Index: resources/sql/autopatches/20140126.diff.1.parentrevisionid.sql
===================================================================
--- /dev/null
+++ resources/sql/autopatches/20140126.diff.1.parentrevisionid.sql
@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_differential.differential_diff
+  DROP COLUMN parentRevisionID;
Index: resources/sql/autopatches/20140126.diff.2.repositoryphid.sql
===================================================================
--- /dev/null
+++ resources/sql/autopatches/20140126.diff.2.repositoryphid.sql
@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_differential.differential_diff
+  ADD COLUMN repositoryPHID VARCHAR(64) COLLATE utf8_bin AFTER authorPHID;
Index: src/applications/differential/conduit/ConduitAPI_differential_creatediff_Method.php
===================================================================
--- src/applications/differential/conduit/ConduitAPI_differential_creatediff_Method.php
+++ src/applications/differential/conduit/ConduitAPI_differential_creatediff_Method.php
@@ -1,8 +1,5 @@
 <?php
 
-/**
- * @group conduit
- */
 final class ConduitAPI_differential_creatediff_Method extends ConduitAPIMethod {
 
   public function getMethodDescription() {
@@ -16,10 +13,10 @@
       'sourcePath'                => 'required string',
       'branch'                    => 'required string',
       'bookmark'                  => 'optional string',
-      'sourceControlSystem'       => 'required enum<svn, git>',
+      'sourceControlSystem'       => 'required enum<svn, git, hg>',
       'sourceControlPath'         => 'required string',
       'sourceControlBaseRevision' => 'required string',
-      'parentRevisionID'          => 'optional revisionid',
+      'parentRevisionID'          => 'deprecated',
       'creationMethod'            => 'optional string',
       'authorPHID'                => 'optional phid',
       'arcanistProject'           => 'optional string',
@@ -28,6 +25,7 @@
         'required enum<none, skip, okay, warn, fail, postponed>',
       'unitStatus'                =>
         'required enum<none, skip, okay, warn, fail, postponed>',
+      'repositoryPHID'            => 'optional phid',
     );
   }
 
@@ -57,20 +55,14 @@
     $diff->setAuthorPHID($request->getValue('authorPHID'));
     $diff->setBookmark($request->getValue('bookmark'));
 
-    $parent_id = $request->getValue('parentRevisionID');
-    if ($parent_id) {
-      // NOTE: If the viewer can't see the parent revision, just don't set
-      // a parent revision ID. This isn't used for anything meaningful.
-      // TODO: Can we delete this entirely?
-      $parent_rev = id(new DifferentialRevisionQuery())
-        ->setViewer($request->getUser())
-        ->withIDs(array($parent_id))
-        ->execute();
-      if ($parent_rev) {
-        $parent_rev = head($parent_rev);
-        if (!$parent_rev->isClosed()) {
-          $diff->setParentRevisionID($parent_id);
-        }
+    $repository_phid = $request->getValue('repositoryPHID');
+    if ($repository_phid) {
+      $repository = id(new PhabricatorRepositoryQuery())
+        ->setViewer($request->getViewer())
+        ->withPHIDs(array($repository_phid))
+        ->executeOne();
+      if ($repository) {
+        $diff->setRepositoryPHID($repository->getPHID());
       }
     }
 
Index: src/applications/differential/storage/DifferentialDiff.php
===================================================================
--- src/applications/differential/storage/DifferentialDiff.php
+++ src/applications/differential/storage/DifferentialDiff.php
@@ -8,6 +8,7 @@
 
   protected $revisionID;
   protected $authorPHID;
+  protected $repositoryPHID;
 
   protected $sourceMachine;
   protected $sourcePath;
@@ -24,7 +25,6 @@
   protected $branch;
   protected $bookmark;
 
-  protected $parentRevisionID;
   protected $arcanistProjectPHID;
   protected $creationMethod;
   protected $repositoryUUID;
@@ -223,7 +223,6 @@
   public function getDiffDict() {
     $dict = array(
       'id' => $this->getID(),
-      'parent' => $this->getParentRevisionID(),
       'revisionID' => $this->getRevisionID(),
       'dateCreated' => $this->getDateCreated(),
       'dateModified' => $this->getDateModified(),