diff --git a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
index eeb4a44ced..5471cd9165 100644
--- a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
+++ b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
@@ -1,927 +1,937 @@
 <?php
 
 /**
  * @task config   Configuring the Hook Engine
  * @task hook     Hook Execution
  * @task git      Git Hooks
  * @task hg       Mercurial Hooks
  * @task svn      Subversion Hooks
  * @task internal Internals
  */
 final class DiffusionCommitHookEngine extends Phobject {
 
   const ENV_USER = 'PHABRICATOR_USER';
   const ENV_REMOTE_ADDRESS = 'PHABRICATOR_REMOTE_ADDRESS';
   const ENV_REMOTE_PROTOCOL = 'PHABRICATOR_REMOTE_PROTOCOL';
 
   const EMPTY_HASH = '0000000000000000000000000000000000000000';
 
   private $viewer;
   private $repository;
   private $stdin;
   private $subversionTransaction;
   private $subversionRepository;
   private $remoteAddress;
   private $remoteProtocol;
   private $transactionKey;
   private $mercurialHook;
+  private $mercurialCommits = array();
 
   private $heraldViewerProjects;
 
 
 /* -(  Config  )------------------------------------------------------------- */
 
 
   public function setRemoteProtocol($remote_protocol) {
     $this->remoteProtocol = $remote_protocol;
     return $this;
   }
 
   public function getRemoteProtocol() {
     return $this->remoteProtocol;
   }
 
   public function setRemoteAddress($remote_address) {
     $this->remoteAddress = $remote_address;
     return $this;
   }
 
   public function getRemoteAddress() {
     return $this->remoteAddress;
   }
 
   private function getRemoteAddressForLog() {
     // If whatever we have here isn't a valid IPv4 address, just store `null`.
     // Older versions of PHP return `-1` on failure instead of `false`.
     $remote_address = $this->getRemoteAddress();
     $remote_address = max(0, ip2long($remote_address));
     $remote_address = nonempty($remote_address, null);
     return $remote_address;
   }
 
   private function getTransactionKey() {
     if (!$this->transactionKey) {
       $entropy = Filesystem::readRandomBytes(64);
       $this->transactionKey = PhabricatorHash::digestForIndex($entropy);
     }
     return $this->transactionKey;
   }
 
   public function setSubversionTransactionInfo($transaction, $repository) {
     $this->subversionTransaction = $transaction;
     $this->subversionRepository = $repository;
     return $this;
   }
 
   public function setStdin($stdin) {
     $this->stdin = $stdin;
     return $this;
   }
 
   public function getStdin() {
     return $this->stdin;
   }
 
   public function setRepository(PhabricatorRepository $repository) {
     $this->repository = $repository;
     return $this;
   }
 
   public function getRepository() {
     return $this->repository;
   }
 
   public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
     return $this;
   }
 
   public function getViewer() {
     return $this->viewer;
   }
 
   public function setMercurialHook($mercurial_hook) {
     $this->mercurialHook = $mercurial_hook;
     return $this;
   }
 
   public function getMercurialHook() {
     return $this->mercurialHook;
   }
 
 
 /* -(  Hook Execution  )----------------------------------------------------- */
 
 
   public function execute() {
     $ref_updates = $this->findRefUpdates();
     $all_updates = $ref_updates;
 
     $caught = null;
     try {
 
       try {
         $this->rejectDangerousChanges($ref_updates);
       } catch (DiffusionCommitHookRejectException $ex) {
         // If we're rejecting dangerous changes, flag everything that we've
         // seen as rejected so it's clear that none of it was accepted.
         foreach ($all_updates as $update) {
           $update->setRejectCode(
             PhabricatorRepositoryPushLog::REJECT_DANGEROUS);
         }
         throw $ex;
       }
 
       $this->applyHeraldRefRules($ref_updates, $all_updates);
 
       $content_updates = $this->findContentUpdates($ref_updates);
       $all_updates = array_merge($all_updates, $content_updates);
 
       $this->applyHeraldContentRules($content_updates, $all_updates);
 
       // TODO: Fire external hooks.
 
       // If we make it this far, we're accepting these changes. Mark all the
       // logs as accepted.
       foreach ($all_updates as $update) {
         $update->setRejectCode(PhabricatorRepositoryPushLog::REJECT_ACCEPT);
       }
     } catch (Exception $ex) {
       // We'll throw this again in a minute, but we want to save all the logs
       // first.
       $caught = $ex;
     }
 
     // Save all the logs no matter what the outcome was.
     foreach ($all_updates as $update) {
       $update->save();
     }
 
     if ($caught) {
       throw $caught;
     }
 
     return 0;
   }
 
   private function findRefUpdates() {
     $type = $this->getRepository()->getVersionControlSystem();
     switch ($type) {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
         return $this->findGitRefUpdates();
       case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
         return $this->findMercurialRefUpdates();
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
         return $this->findSubversionRefUpdates();
       default:
         throw new Exception(pht('Unsupported repository type "%s"!', $type));
     }
   }
 
   private function rejectDangerousChanges(array $ref_updates) {
     assert_instances_of($ref_updates, 'PhabricatorRepositoryPushLog');
 
     $repository = $this->getRepository();
     if ($repository->shouldAllowDangerousChanges()) {
       return;
     }
 
     $flag_dangerous = PhabricatorRepositoryPushLog::CHANGEFLAG_DANGEROUS;
 
     foreach ($ref_updates as $ref_update) {
       if (!$ref_update->hasChangeFlags($flag_dangerous)) {
         // This is not a dangerous change.
         continue;
       }
 
       // We either have a branch deletion or a non fast-forward branch update.
       // Format a message and reject the push.
 
       $message = pht(
         "DANGEROUS CHANGE: %s\n".
         "Dangerous change protection is enabled for this repository.\n".
         "Edit the repository configuration before making dangerous changes.",
         $ref_update->getDangerousChangeDescription());
 
       throw new DiffusionCommitHookRejectException($message);
     }
   }
 
   private function findContentUpdates(array $ref_updates) {
     assert_instances_of($ref_updates, 'PhabricatorRepositoryPushLog');
 
     $type = $this->getRepository()->getVersionControlSystem();
     switch ($type) {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
         return $this->findGitContentUpdates($ref_updates);
       case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
         return $this->findMercurialContentUpdates($ref_updates);
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
         return $this->findSubversionContentUpdates($ref_updates);
       default:
         throw new Exception(pht('Unsupported repository type "%s"!', $type));
     }
   }
 
 
 /* -(  Herald  )------------------------------------------------------------- */
 
   private function applyHeraldRefRules(
     array $ref_updates,
     array $all_updates) {
     $this->applyHeraldRules(
       $ref_updates,
       new HeraldPreCommitRefAdapter(),
       $all_updates);
   }
 
   private function applyHeraldContentRules(
     array $content_updates,
     array $all_updates) {
     $this->applyHeraldRules(
       $content_updates,
       new HeraldPreCommitContentAdapter(),
       $all_updates);
   }
 
   private function applyHeraldRules(
     array $updates,
     HeraldAdapter $adapter_template,
     array $all_updates) {
 
     if (!$updates) {
       return;
     }
 
     $adapter_template->setHookEngine($this);
 
     $engine = new HeraldEngine();
     $rules = null;
     $blocking_effect = null;
     foreach ($updates as $update) {
       $adapter = id(clone $adapter_template)
         ->setPushLog($update);
 
       if ($rules === null) {
         $rules = $engine->loadRulesForAdapter($adapter);
       }
 
       $effects = $engine->applyRules($rules, $adapter);
       $engine->applyEffects($effects, $adapter, $rules);
       $xscript = $engine->getTranscript();
 
       if ($blocking_effect === null) {
         foreach ($effects as $effect) {
           if ($effect->getAction() == HeraldAdapter::ACTION_BLOCK) {
             $blocking_effect = $effect;
             break;
           }
         }
       }
     }
 
     if ($blocking_effect) {
       foreach ($all_updates as $update) {
         $update->setRejectCode(PhabricatorRepositoryPushLog::REJECT_HERALD);
         $update->setRejectDetails($blocking_effect->getRulePHID());
       }
 
       $message = $blocking_effect->getTarget();
       if (!strlen($message)) {
         $message = pht('(None.)');
       }
 
       $rules = mpull($rules, null, 'getID');
       $rule = idx($rules, $effect->getRuleID());
       if ($rule && strlen($rule->getName())) {
         $rule_name = $rule->getName();
       } else {
         $rule_name = pht('Unnamed Herald Rule');
       }
 
       throw new DiffusionCommitHookRejectException(
         pht(
           "This commit was rejected by Herald pre-commit rule %s.\n".
           "Rule: %s\n".
           "Reason: %s",
           'H'.$blocking_effect->getRuleID(),
           $rule_name,
           $message));
     }
   }
 
   public function loadViewerProjectPHIDsForHerald() {
     // This just caches the viewer's projects so we don't need to load them
     // over and over again when applying Herald rules.
     if ($this->heraldViewerProjects === null) {
       $this->heraldViewerProjects = id(new PhabricatorProjectQuery())
         ->setViewer($this->getViewer())
         ->withMemberPHIDs(array($this->getViewer()->getPHID()))
         ->execute();
     }
 
     return mpull($this->heraldViewerProjects, 'getPHID');
   }
 
 
 /* -(  Git  )---------------------------------------------------------------- */
 
 
   private function findGitRefUpdates() {
     $ref_updates = array();
 
     // First, parse stdin, which lists all the ref changes. The input looks
     // like this:
     //
     //   <old hash> <new hash> <ref>
 
     $stdin = $this->getStdin();
     $lines = phutil_split_lines($stdin, $retain_endings = false);
     foreach ($lines as $line) {
       $parts = explode(' ', $line, 3);
       if (count($parts) != 3) {
         throw new Exception(pht('Expected "old new ref", got "%s".', $line));
       }
 
       $ref_old = $parts[0];
       $ref_new = $parts[1];
       $ref_raw = $parts[2];
 
       if (preg_match('(^refs/heads/)', $ref_raw)) {
         $ref_type = PhabricatorRepositoryPushLog::REFTYPE_BRANCH;
         $ref_raw = substr($ref_raw, strlen('refs/heads/'));
       } else if (preg_match('(^refs/tags/)', $ref_raw)) {
         $ref_type = PhabricatorRepositoryPushLog::REFTYPE_TAG;
         $ref_raw = substr($ref_raw, strlen('refs/tags/'));
       } else {
         throw new Exception(
           pht(
             "Unable to identify the reftype of '%s'. Rejecting push.",
             $ref_raw));
       }
 
       $ref_update = $this->newPushLog()
         ->setRefType($ref_type)
         ->setRefName($ref_raw)
         ->setRefOld($ref_old)
         ->setRefNew($ref_new);
 
       $ref_updates[] = $ref_update;
     }
 
     $this->findGitMergeBases($ref_updates);
     $this->findGitChangeFlags($ref_updates);
 
     return $ref_updates;
   }
 
 
   private function findGitMergeBases(array $ref_updates) {
     assert_instances_of($ref_updates, 'PhabricatorRepositoryPushLog');
 
     $futures = array();
     foreach ($ref_updates as $key => $ref_update) {
       // If the old hash is "00000...", the ref is being created (either a new
       // branch, or a new tag). If the new hash is "00000...", the ref is being
       // deleted. If both are nonempty, the ref is being updated. For updates,
       // we'll figure out the `merge-base` of the old and new objects here. This
       // lets us reject non-FF changes cheaply; later, we'll figure out exactly
       // which commits are new.
       $ref_old = $ref_update->getRefOld();
       $ref_new = $ref_update->getRefNew();
 
       if (($ref_old === self::EMPTY_HASH) ||
           ($ref_new === self::EMPTY_HASH)) {
         continue;
       }
 
       $futures[$key] = $this->getRepository()->getLocalCommandFuture(
         'merge-base %s %s',
         $ref_old,
         $ref_new);
     }
 
     foreach (Futures($futures)->limit(8) as $key => $future) {
 
       // If 'old' and 'new' have no common ancestors (for example, a force push
       // which completely rewrites a ref), `git merge-base` will exit with
       // an error and no output. It would be nice to find a positive test
       // for this instead, but I couldn't immediately come up with one. See
       // T4224. Assume this means there are no ancestors.
 
       list($err, $stdout) = $future->resolve();
 
       if ($err) {
         $merge_base = null;
       } else {
         $merge_base = rtrim($stdout, "\n");
       }
 
       $ref_update->setMergeBase($merge_base);
     }
 
     return $ref_updates;
   }
 
 
   private function findGitChangeFlags(array $ref_updates) {
     assert_instances_of($ref_updates, 'PhabricatorRepositoryPushLog');
 
     foreach ($ref_updates as $key => $ref_update) {
       $ref_old = $ref_update->getRefOld();
       $ref_new = $ref_update->getRefNew();
       $ref_type = $ref_update->getRefType();
 
       $ref_flags = 0;
       $dangerous = null;
 
       if ($ref_old === self::EMPTY_HASH) {
         $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_ADD;
       } else if ($ref_new === self::EMPTY_HASH) {
         $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_DELETE;
         if ($ref_type == PhabricatorRepositoryPushLog::REFTYPE_BRANCH) {
           $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_DANGEROUS;
           $dangerous = pht(
             "The change you're attempting to push deletes the branch '%s'.",
             $ref_update->getRefName());
         }
       } else {
         $merge_base = $ref_update->getMergeBase();
         if ($merge_base == $ref_old) {
           // This is a fast-forward update to an existing branch.
           // These are safe.
           $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_APPEND;
         } else {
           $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_REWRITE;
 
           // For now, we don't consider deleting or moving tags to be a
           // "dangerous" update. It's way harder to get wrong and should be easy
           // to recover from once we have better logging. Only add the dangerous
           // flag if this ref is a branch.
 
           if ($ref_type == PhabricatorRepositoryPushLog::REFTYPE_BRANCH) {
             $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_DANGEROUS;
 
             $dangerous = pht(
               "The change you're attempting to push updates the branch '%s' ".
               "from '%s' to '%s', but this is not a fast-forward. Pushes ".
               "which rewrite published branch history are dangerous.",
               $ref_update->getRefName(),
               $ref_update->getRefOldShort(),
               $ref_update->getRefNewShort());
           }
         }
       }
 
       $ref_update->setChangeFlags($ref_flags);
       if ($dangerous !== null) {
         $ref_update->attachDangerousChangeDescription($dangerous);
       }
     }
 
     return $ref_updates;
   }
 
 
   private function findGitContentUpdates(array $ref_updates) {
     $flag_delete = PhabricatorRepositoryPushLog::CHANGEFLAG_DELETE;
 
     $futures = array();
     foreach ($ref_updates as $key => $ref_update) {
       if ($ref_update->hasChangeFlags($flag_delete)) {
         // Deleting a branch or tag can never create any new commits.
         continue;
       }
 
       // NOTE: This piece of magic finds all new commits, by walking backward
       // from the new value to the value of *any* existing ref in the
       // repository. Particularly, this will cover the cases of a new branch, a
       // completely moved tag, etc.
       $futures[$key] = $this->getRepository()->getLocalCommandFuture(
         'log --format=%s %s --not --all',
         '%H',
         $ref_update->getRefNew());
     }
 
     $content_updates = array();
     foreach (Futures($futures)->limit(8) as $key => $future) {
       list($stdout) = $future->resolvex();
 
       if (!strlen(trim($stdout))) {
         // This change doesn't have any new commits. One common case of this
         // is creating a new tag which points at an existing commit.
         continue;
       }
 
       $commits = phutil_split_lines($stdout, $retain_newlines = false);
 
       foreach ($commits as $commit) {
         $content_updates[$commit] = $this->newPushLog()
           ->setRefType(PhabricatorRepositoryPushLog::REFTYPE_COMMIT)
           ->setRefNew($commit)
           ->setChangeFlags(PhabricatorRepositoryPushLog::CHANGEFLAG_ADD);
       }
     }
 
     return $content_updates;
   }
 
 
 /* -(  Mercurial  )---------------------------------------------------------- */
 
 
   private function findMercurialRefUpdates() {
     $hook = $this->getMercurialHook();
     switch ($hook) {
       case 'pretxnchangegroup':
         return $this->findMercurialChangegroupRefUpdates();
       case 'prepushkey':
         return $this->findMercurialPushKeyRefUpdates();
       default:
         throw new Exception(pht('Unrecognized hook "%s"!', $hook));
     }
   }
 
   private function findMercurialChangegroupRefUpdates() {
     $hg_node = getenv('HG_NODE');
     if (!$hg_node) {
       throw new Exception(pht('Expected HG_NODE in environment!'));
     }
 
     // NOTE: We need to make sure this is passed to subprocesses, or they won't
     // be able to see new commits. Mercurial uses this as a marker to determine
     // whether the pending changes are visible or not.
     $_ENV['HG_PENDING'] = getenv('HG_PENDING');
     $repository = $this->getRepository();
 
     $futures = array();
 
     foreach (array('old', 'new') as $key) {
       $futures[$key] = $repository->getLocalCommandFuture(
         'heads --template %s',
         '{node}\1{branches}\2');
     }
     // Wipe HG_PENDING out of the old environment so we see the pre-commit
     // state of the repository.
     $futures['old']->updateEnv('HG_PENDING', null);
 
     $futures['commits'] = $repository->getLocalCommandFuture(
       "log --rev %s --rev tip --template %s",
       hgsprintf('%s', $hg_node),
       '{node}\1{branches}\2');
 
     // Resolve all of the futures now. We don't need the 'commits' future yet,
     // but it simplifies the logic to just get it out of the way.
     foreach (Futures($futures) as $future) {
       $future->resolvex();
     }
 
     list($commit_raw) = $futures['commits']->resolvex();
     $commit_map = $this->parseMercurialCommits($commit_raw);
+    $this->mercurialCommits = $commit_map;
 
     list($old_raw) = $futures['old']->resolvex();
     $old_refs = $this->parseMercurialHeads($old_raw);
 
     list($new_raw) = $futures['new']->resolvex();
     $new_refs = $this->parseMercurialHeads($new_raw);
 
     $all_refs = array_keys($old_refs + $new_refs);
 
     $ref_updates = array();
     foreach ($all_refs as $ref) {
       $old_heads = idx($old_refs, $ref, array());
       $new_heads = idx($new_refs, $ref, array());
 
       sort($old_heads);
       sort($new_heads);
 
       if ($old_heads === $new_heads) {
         // No changes to this branch, so skip it.
         continue;
       }
 
       if (!$new_heads) {
         if ($old_heads) {
           // It looks like this push deletes a branch, but that isn't possible
           // in Mercurial, so something is going wrong here. Bail out.
           throw new Exception(
             pht(
               'Mercurial repository has no new head for branch "%s" after '.
               'push. This is unexpected; rejecting change.'));
         } else {
           // Obviously, this should never be possible either, as it makes
           // no sense. Explode.
           throw new Exception(
             pht(
               'Mercurial repository has no new or old heads for branch "%s" '.
               'after push. This makes no sense; rejecting change.'));
         }
       }
 
       $stray_heads = array();
       if (count($old_heads) > 1) {
         // HORRIBLE: In Mercurial, branches can have multiple heads. If the
         // old branch had multiple heads, we need to figure out which new
         // heads descend from which old heads, so we can tell whether you're
         // actively creating new heads (dangerous) or just working in a
         // repository that's already full of garbage (strongly discouraged but
         // not as inherently dangerous). These cases should be very uncommon.
 
         $dfutures = array();
         foreach ($old_heads as $old_head) {
           $dfutures[$old_head] = $repository->getLocalCommandFuture(
             'log --rev %s --template %s',
             hgsprintf('(descendants(%s) and head())', $old_head),
             '{node}\1');
         }
 
         $head_map = array();
         foreach (Futures($dfutures) as $future_head => $dfuture) {
           list($stdout) = $dfuture->resolvex();
           $head_map[$future_head] = array_filter(explode("\1", $stdout));
         }
 
         // Now, find all the new stray heads this push creates, if any. These
         // are new heads which do not descend from the old heads.
         $seen = array_fuse(array_mergev($head_map));
         foreach ($new_heads as $new_head) {
           if (empty($seen[$new_head])) {
             $head_map[self::EMPTY_HASH][] = $new_head;
           }
         }
       } else if ($old_heads) {
         $head_map[head($old_heads)] = $new_heads;
       } else {
         $head_map[self::EMPTY_HASH] = $new_heads;
       }
 
       foreach ($head_map as $old_head => $child_heads) {
         foreach ($child_heads as $new_head) {
           if ($new_head === $old_head) {
             continue;
           }
 
           $ref_flags = 0;
           $dangerous = null;
           if ($old_head == self::EMPTY_HASH) {
             $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_ADD;
           } else {
             $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_APPEND;
           }
 
           $splits_existing_head = (count($child_heads) > 1);
           $creates_duplicate_head = ($old_head == self::EMPTY_HASH) &&
                                     (count($head_map) > 1);
 
           if ($splits_existing_head || $creates_duplicate_head) {
             $readable_child_heads = array();
             foreach ($child_heads as $child_head) {
               $readable_child_heads[] = substr($child_head, 0, 12);
             }
 
             $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_DANGEROUS;
 
             if ($splits_existing_head) {
               // We're splitting an existing head into two or more heads.
               // This is dangerous, and a super bad idea. Note that we're only
               // raising this if you're actively splitting a branch head. If a
               // head split in the past, we don't consider appends to it
               // to be dangerous.
               $dangerous = pht(
                 "The change you're attempting to push splits the head of ".
                 "branch '%s' into multiple heads: %s. This is inadvisable ".
                 "and dangerous.",
                 $ref,
                 implode(', ', $readable_child_heads));
             } else {
               // We're adding a second (or more) head to a branch. The new
               // head is not a descendant of any old head.
               $dangerous = pht(
                 "The change you're attempting to push creates new, divergent ".
                 "heads for the branch '%s': %s. This is inadvisable and ".
                 "dangerous.",
                 $ref,
                 implode(', ', $readable_child_heads));
             }
           }
 
           $ref_update = $this->newPushLog()
             ->setRefType(PhabricatorRepositoryPushLog::REFTYPE_BRANCH)
             ->setRefName($ref)
             ->setRefOld($old_head)
             ->setRefNew($new_head)
             ->setChangeFlags($ref_flags);
 
           if ($dangerous !== null) {
             $ref_update->attachDangerousChangeDescription($dangerous);
           }
 
           $ref_updates[] = $ref_update;
         }
       }
     }
 
     return $ref_updates;
   }
 
   private function findMercurialPushKeyRefUpdates() {
     $key_namespace = getenv('HG_NAMESPACE');
 
     if ($key_namespace === 'phases') {
       // Mercurial changes commit phases as part of normal push operations. We
       // just ignore these, as they don't seem to represent anything
       // interesting.
       return array();
     }
 
     $key_name = getenv('HG_KEY');
 
     $key_old = getenv('HG_OLD');
     if (!strlen($key_old)) {
       $key_old = null;
     }
 
     $key_new = getenv('HG_NEW');
     if (!strlen($key_new)) {
       $key_new = null;
     }
 
     if ($key_namespace !== 'bookmarks') {
       throw new Exception(
         pht(
           "Unknown Mercurial key namespace '%s', with key '%s' (%s -> %s). ".
           "Rejecting push.",
           $key_namespace,
           $key_name,
           coalesce($key_old, pht('null')),
           coalesce($key_new, pht('null'))));
     }
 
     if ($key_old === $key_new) {
       // We get a callback when the bookmark doesn't change. Just ignore this,
       // as it's a no-op.
       return array();
     }
 
     $ref_flags = 0;
     $merge_base = null;
     if ($key_old === null) {
       $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_ADD;
     } else if ($key_new === null) {
       $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_DELETE;
     } else {
       list($merge_base_raw) = $this->getRepository()->execxLocalCommand(
         'log --template %s --rev %s',
         '{node}',
         hgsprintf('ancestor(%s, %s)', $key_old, $key_new));
 
       if (strlen(trim($merge_base_raw))) {
         $merge_base = trim($merge_base_raw);
       }
 
       if ($merge_base && ($merge_base === $key_old)) {
         $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_APPEND;
       } else {
         $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_REWRITE;
       }
     }
 
     $ref_update = $this->newPushLog()
       ->setRefType(PhabricatorRepositoryPushLog::REFTYPE_BOOKMARK)
       ->setRefName($key_name)
       ->setRefOld(coalesce($key_old, self::EMPTY_HASH))
       ->setRefNew(coalesce($key_new, self::EMPTY_HASH))
       ->setChangeFlags($ref_flags);
 
     return array($ref_update);
   }
 
   private function findMercurialContentUpdates(array $ref_updates) {
-    // TODO: Implement.
-    return array();
+    $content_updates = array();
+
+    foreach ($this->mercurialCommits as $commit => $branches) {
+      $content_updates[$commit] = $this->newPushLog()
+        ->setRefType(PhabricatorRepositoryPushLog::REFTYPE_COMMIT)
+        ->setRefNew($commit)
+        ->setChangeFlags(PhabricatorRepositoryPushLog::CHANGEFLAG_ADD);
+    }
+
+    return $content_updates;
   }
 
   private function parseMercurialCommits($raw) {
     $commits_lines = explode("\2", $raw);
     $commits_lines = array_filter($commits_lines);
     $commit_map = array();
     foreach ($commits_lines as $commit_line) {
       list($node, $branches_raw) = explode("\1", $commit_line);
 
       if (!strlen($branches_raw)) {
         $branches = array('default');
       } else {
         $branches = explode(' ', $branches_raw);
       }
 
       $commit_map[$node] = $branches;
     }
 
     return $commit_map;
   }
 
   private function parseMercurialHeads($raw) {
     $heads_map = $this->parseMercurialCommits($raw);
 
     $heads = array();
     foreach ($heads_map as $commit => $branches) {
       foreach ($branches as $branch) {
         $heads[$branch][] = $commit;
       }
     }
 
     return $heads;
   }
 
 
 /* -(  Subversion  )--------------------------------------------------------- */
 
 
   private function findSubversionRefUpdates() {
     // Subversion doesn't have any kind of mutable ref metadata.
     return array();
   }
 
   private function findSubversionContentUpdates(array $ref_updates) {
     list($youngest) = execx(
       'svnlook youngest %s',
       $this->subversionRepository);
     $ref_new = (int)$youngest + 1;
 
     $ref_flags = 0;
     $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_ADD;
     $ref_flags |= PhabricatorRepositoryPushLog::CHANGEFLAG_APPEND;
 
     $ref_content = $this->newPushLog()
       ->setRefType(PhabricatorRepositoryPushLog::REFTYPE_COMMIT)
       ->setRefNew($ref_new)
       ->setChangeFlags($ref_flags);
 
     return array($ref_content);
   }
 
 
 /* -(  Internals  )---------------------------------------------------------- */
 
 
   private function newPushLog() {
     // NOTE: By default, we create these with REJECT_BROKEN as the reject
     // code. This indicates a broken hook, and covers the case where we
     // encounter some unexpected exception and consequently reject the changes.
 
     // NOTE: We generate PHIDs up front so the Herald transcripts can pick them
     // up.
     $phid = id(new PhabricatorRepositoryPushLog())->generatePHID();
 
     return PhabricatorRepositoryPushLog::initializeNewLog($this->getViewer())
       ->setPHID($phid)
       ->attachRepository($this->getRepository())
       ->setRepositoryPHID($this->getRepository()->getPHID())
       ->setEpoch(time())
       ->setRemoteAddress($this->getRemoteAddressForLog())
       ->setRemoteProtocol($this->getRemoteProtocol())
       ->setTransactionKey($this->getTransactionKey())
       ->setRejectCode(PhabricatorRepositoryPushLog::REJECT_BROKEN)
       ->setRejectDetails(null);
   }
 
   public function loadChangesetsForCommit($identifier) {
     $vcs = $this->getRepository()->getVersionControlSystem();
     switch ($vcs) {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
       case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
         // For git and hg, we can use normal commands.
         $drequest = DiffusionRequest::newFromDictionary(
           array(
             'repository' => $this->getRepository(),
             'user' => $this->getViewer(),
             'commit' => $identifier,
           ));
         $raw_diff = DiffusionRawDiffQuery::newFromDiffusionRequest($drequest)
           ->setTimeout(5 * 60)
           ->setLinesOfContext(0)
           ->loadRawDiff();
         break;
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
         // TODO: This diff has 3 lines of context, which produces slightly
         // incorrect "added file content" and "removed file content" results.
         // This may also choke on binaries, but "svnlook diff" does not support
         // the "--diff-cmd" flag.
 
         // For subversion, we need to use `svnlook`.
         list($raw_diff) = execx(
           'svnlook diff -t %s %s',
           $this->subversionTransaction,
           $this->subversionRepository);
         break;
       default:
         throw new Exception(pht("Unknown VCS '%s!'", $vcs));
     }
 
     $parser = new ArcanistDiffParser();
     $changes = $parser->parseDiff($raw_diff);
     $diff = DifferentialDiff::newFromRawChanges($changes);
     return $diff->getChangesets();
   }
 
 }