diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -570,6 +570,8 @@
     'DiffusionCachedResolveRefsQuery' => 'applications/diffusion/query/DiffusionCachedResolveRefsQuery.php',
     'DiffusionChangeController' => 'applications/diffusion/controller/DiffusionChangeController.php',
     'DiffusionChangeHeraldFieldGroup' => 'applications/diffusion/herald/DiffusionChangeHeraldFieldGroup.php',
+    'DiffusionCommandEngine' => 'applications/diffusion/protocol/DiffusionCommandEngine.php',
+    'DiffusionCommandEngineTestCase' => 'applications/diffusion/protocol/__tests__/DiffusionCommandEngineTestCase.php',
     'DiffusionCommitAffectedFilesHeraldField' => 'applications/diffusion/herald/DiffusionCommitAffectedFilesHeraldField.php',
     'DiffusionCommitAuthorHeraldField' => 'applications/diffusion/herald/DiffusionCommitAuthorHeraldField.php',
     'DiffusionCommitAutocloseHeraldField' => 'applications/diffusion/herald/DiffusionCommitAutocloseHeraldField.php',
@@ -634,6 +636,7 @@
     'DiffusionGitBlameQuery' => 'applications/diffusion/query/blame/DiffusionGitBlameQuery.php',
     'DiffusionGitBranch' => 'applications/diffusion/data/DiffusionGitBranch.php',
     'DiffusionGitBranchTestCase' => 'applications/diffusion/data/__tests__/DiffusionGitBranchTestCase.php',
+    'DiffusionGitCommandEngine' => 'applications/diffusion/protocol/DiffusionGitCommandEngine.php',
     'DiffusionGitFileContentQuery' => 'applications/diffusion/query/filecontent/DiffusionGitFileContentQuery.php',
     'DiffusionGitLFSAuthenticateWorkflow' => 'applications/diffusion/gitlfs/DiffusionGitLFSAuthenticateWorkflow.php',
     'DiffusionGitLFSResponse' => 'applications/diffusion/response/DiffusionGitLFSResponse.php',
@@ -667,6 +670,7 @@
     'DiffusionLowLevelQuery' => 'applications/diffusion/query/lowlevel/DiffusionLowLevelQuery.php',
     'DiffusionLowLevelResolveRefsQuery' => 'applications/diffusion/query/lowlevel/DiffusionLowLevelResolveRefsQuery.php',
     'DiffusionMercurialBlameQuery' => 'applications/diffusion/query/blame/DiffusionMercurialBlameQuery.php',
+    'DiffusionMercurialCommandEngine' => 'applications/diffusion/protocol/DiffusionMercurialCommandEngine.php',
     'DiffusionMercurialFileContentQuery' => 'applications/diffusion/query/filecontent/DiffusionMercurialFileContentQuery.php',
     'DiffusionMercurialRawDiffQuery' => 'applications/diffusion/query/rawdiff/DiffusionMercurialRawDiffQuery.php',
     'DiffusionMercurialRequest' => 'applications/diffusion/request/DiffusionMercurialRequest.php',
@@ -788,6 +792,7 @@
     'DiffusionServeController' => 'applications/diffusion/controller/DiffusionServeController.php',
     'DiffusionSetPasswordSettingsPanel' => 'applications/diffusion/panel/DiffusionSetPasswordSettingsPanel.php',
     'DiffusionSetupException' => 'applications/diffusion/exception/DiffusionSetupException.php',
+    'DiffusionSubversionCommandEngine' => 'applications/diffusion/protocol/DiffusionSubversionCommandEngine.php',
     'DiffusionSubversionSSHWorkflow' => 'applications/diffusion/ssh/DiffusionSubversionSSHWorkflow.php',
     'DiffusionSubversionServeSSHWorkflow' => 'applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php',
     'DiffusionSubversionWireProtocol' => 'applications/diffusion/protocol/DiffusionSubversionWireProtocol.php',
@@ -4771,6 +4776,8 @@
     'DiffusionCachedResolveRefsQuery' => 'DiffusionLowLevelQuery',
     'DiffusionChangeController' => 'DiffusionController',
     'DiffusionChangeHeraldFieldGroup' => 'HeraldFieldGroup',
+    'DiffusionCommandEngine' => 'Phobject',
+    'DiffusionCommandEngineTestCase' => 'PhabricatorTestCase',
     'DiffusionCommitAffectedFilesHeraldField' => 'DiffusionCommitHeraldField',
     'DiffusionCommitAuthorHeraldField' => 'DiffusionCommitHeraldField',
     'DiffusionCommitAutocloseHeraldField' => 'DiffusionCommitHeraldField',
@@ -4835,6 +4842,7 @@
     'DiffusionGitBlameQuery' => 'DiffusionBlameQuery',
     'DiffusionGitBranch' => 'Phobject',
     'DiffusionGitBranchTestCase' => 'PhabricatorTestCase',
+    'DiffusionGitCommandEngine' => 'DiffusionCommandEngine',
     'DiffusionGitFileContentQuery' => 'DiffusionFileContentQuery',
     'DiffusionGitLFSAuthenticateWorkflow' => 'DiffusionGitSSHWorkflow',
     'DiffusionGitLFSResponse' => 'AphrontResponse',
@@ -4868,6 +4876,7 @@
     'DiffusionLowLevelQuery' => 'Phobject',
     'DiffusionLowLevelResolveRefsQuery' => 'DiffusionLowLevelQuery',
     'DiffusionMercurialBlameQuery' => 'DiffusionBlameQuery',
+    'DiffusionMercurialCommandEngine' => 'DiffusionCommandEngine',
     'DiffusionMercurialFileContentQuery' => 'DiffusionFileContentQuery',
     'DiffusionMercurialRawDiffQuery' => 'DiffusionRawDiffQuery',
     'DiffusionMercurialRequest' => 'DiffusionRequest',
@@ -4989,6 +4998,7 @@
     'DiffusionServeController' => 'DiffusionController',
     'DiffusionSetPasswordSettingsPanel' => 'PhabricatorSettingsPanel',
     'DiffusionSetupException' => 'Exception',
+    'DiffusionSubversionCommandEngine' => 'DiffusionCommandEngine',
     'DiffusionSubversionSSHWorkflow' => 'DiffusionSSHWorkflow',
     'DiffusionSubversionServeSSHWorkflow' => 'DiffusionSubversionSSHWorkflow',
     'DiffusionSubversionWireProtocol' => 'Phobject',
diff --git a/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php b/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
--- a/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
+++ b/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php
@@ -131,7 +131,8 @@
       hgsprintf('reverse(ancestors(%s))', $commit_hash),
       $path_arg);
 
-    $stdout = PhabricatorRepository::filterMercurialDebugOutput($stdout);
+    $stdout = DiffusionMercurialCommandEngine::filterMercurialDebugOutput(
+      $stdout);
     $lines = explode("\n", trim($stdout));
     $lines = array_slice($lines, $offset);
 
diff --git a/src/applications/diffusion/protocol/DiffusionCommandEngine.php b/src/applications/diffusion/protocol/DiffusionCommandEngine.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/protocol/DiffusionCommandEngine.php
@@ -0,0 +1,173 @@
+<?php
+
+abstract class DiffusionCommandEngine extends Phobject {
+
+  private $repository;
+  private $protocol;
+  private $credentialPHID;
+  private $argv;
+  private $passthru;
+
+  public static function newCommandEngine(PhabricatorRepository $repository) {
+    $engines = self::newCommandEngines();
+
+    foreach ($engines as $engine) {
+      if ($engine->canBuildForRepository($repository)) {
+        return id(clone $engine)
+          ->setRepository($repository);
+      }
+    }
+
+    throw new Exception(
+      pht(
+        'No registered command engine can build commands for this '.
+        'repository ("%s").',
+        $repository->getDisplayName()));
+  }
+
+  private static function newCommandEngines() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->execute();
+  }
+
+  abstract protected function canBuildForRepository(
+    PhabricatorRepository $repository);
+
+  abstract protected function newFormattedCommand($pattern, array $argv);
+  abstract protected function newCustomEnvironment();
+
+  public function setRepository(PhabricatorRepository $repository) {
+    $this->repository = $repository;
+    return $this;
+  }
+
+  public function getRepository() {
+    return $this->repository;
+  }
+
+  public function setProtocol($protocol) {
+    $this->protocol = $protocol;
+    return $this;
+  }
+
+  public function getProtocol() {
+    return $this->protocol;
+  }
+
+  public function setCredentialPHID($credential_phid) {
+    $this->credentialPHID = $credential_phid;
+    return $this;
+  }
+
+  public function getCredentialPHID() {
+    return $this->credentialPHID;
+  }
+
+  public function setArgv(array $argv) {
+    $this->argv = $argv;
+    return $this;
+  }
+
+  public function getArgv() {
+    return $this->argv;
+  }
+
+  public function setPassthru($passthru) {
+    $this->passthru = $passthru;
+    return $this;
+  }
+
+  public function getPassthru() {
+    return $this->passthru;
+  }
+
+  public function newFuture() {
+    $argv = $this->newCommandArgv();
+    $env = $this->newCommandEnvironment();
+
+    if ($this->getPassthru()) {
+      $future = newv('PhutilExecPassthru', $argv);
+    } else {
+      $future = newv('ExecFuture', $argv);
+    }
+
+    $future->setEnv($env);
+
+    return $future;
+  }
+
+  private function newCommandArgv() {
+    $argv = $this->argv;
+    $pattern = $argv[0];
+    $argv = array_slice($argv, 1);
+
+    list($pattern, $argv) = $this->newFormattedCommand($pattern, $argv);
+
+    return array_merge(array($pattern), $argv);
+  }
+
+  private function newCommandEnvironment() {
+    $env = $this->newCommonEnvironment() + $this->newCustomEnvironment();
+    foreach ($env as $key => $value) {
+      if ($value === null) {
+        unset($env[$key]);
+      }
+    }
+    return $env;
+  }
+
+  private function newCommonEnvironment() {
+    $env = array();
+      // NOTE: Force the language to "en_US.UTF-8", which overrides locale
+      // settings. This makes stuff print in English instead of, e.g., French,
+      // so we can parse the output of some commands, error messages, etc.
+    $env['LANG'] = 'en_US.UTF-8';
+
+      // Propagate PHABRICATOR_ENV explicitly. For discussion, see T4155.
+    $env['PHABRICATOR_ENV'] = PhabricatorEnv::getSelectedEnvironmentName();
+
+    if ($this->isAnySSHProtocol()) {
+      $credential_phid = $this->getCredentialPHID();
+      if ($credential_phid) {
+        $env['PHABRICATOR_CREDENTIAL'] = $credential_phid;
+      }
+    }
+
+    return $env;
+  }
+
+  protected function isSSHProtocol() {
+    return ($this->getProtocol() == 'ssh');
+  }
+
+  protected function isSVNProtocol() {
+    return ($this->getProtocol() == 'svn');
+  }
+
+  protected function isSVNSSHProtocol() {
+    return ($this->getProtocol() == 'svn+ssh');
+  }
+
+  protected function isHTTPProtocol() {
+    return ($this->getProtocol() == 'http');
+  }
+
+  protected function isHTTPSProtocol() {
+    return ($this->getProtocol() == 'https');
+  }
+
+  protected function isAnyHTTPProtocol() {
+    return ($this->isHTTPProtocol() || $this->isHTTPSProtocol());
+  }
+
+  protected function isAnySSHProtocol() {
+    return ($this->isSSHProtocol() || $this->isSVNSSHProtocol());
+  }
+
+  protected function getSSHWrapper() {
+    $root = dirname(phutil_get_library_root('phabricator'));
+    return $root.'/bin/ssh-connect';
+  }
+
+}
diff --git a/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php b/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php
@@ -0,0 +1,37 @@
+<?php
+
+final class DiffusionGitCommandEngine
+  extends DiffusionCommandEngine {
+
+  protected function canBuildForRepository(
+    PhabricatorRepository $repository) {
+    return $repository->isGit();
+  }
+
+  protected function newFormattedCommand($pattern, array $argv) {
+    $pattern = "git {$pattern}";
+    return array($pattern, $argv);
+  }
+
+  protected function newCustomEnvironment() {
+    $env = array();
+
+    // NOTE: See T2965. Some time after Git 1.7.5.4, Git started fataling if
+    // it can not read $HOME. For many users, $HOME points at /root (this
+    // seems to be a default result of Apache setup). Instead, explicitly
+    // point $HOME at a readable, empty directory so that Git looks for the
+    // config file it's after, fails to locate it, and moves on. This is
+    // really silly, but seems like the least damaging approach to
+    // mitigating the issue.
+
+    $root = dirname(phutil_get_library_root('phabricator'));
+    $env['HOME'] = $root.'/support/empty/';
+
+    if ($this->isAnySSHProtocol()) {
+      $env['GIT_SSH'] = $this->getSSHWrapper();
+    }
+
+    return $env;
+  }
+
+}
diff --git a/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php b/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/protocol/DiffusionMercurialCommandEngine.php
@@ -0,0 +1,70 @@
+<?php
+
+final class DiffusionMercurialCommandEngine
+  extends DiffusionCommandEngine {
+
+  protected function canBuildForRepository(
+    PhabricatorRepository $repository) {
+    return $repository->isHg();
+  }
+
+  protected function newFormattedCommand($pattern, array $argv) {
+    $args = array();
+
+    if ($this->isAnySSHProtocol()) {
+      $pattern = "hg --config ui.ssh=%s {$pattern}";
+      $args[] = $this->getSSHWrapper();
+    } else {
+      $pattern = "hg {$pattern}";
+    }
+
+    return array($pattern, array_merge($args, $argv));
+  }
+
+  protected function newCustomEnvironment() {
+    $env = array();
+
+    // NOTE: This overrides certain configuration, extensions, and settings
+    // which make Mercurial commands do random unusual things.
+    $env['HGPLAIN'] = 1;
+
+    return $env;
+  }
+
+  /**
+   * Sanitize output of an `hg` command invoked with the `--debug` flag to make
+   * it usable.
+   *
+   * @param string Output from `hg --debug ...`
+   * @return string Usable output.
+   */
+  public static function filterMercurialDebugOutput($stdout) {
+    // When hg commands are run with `--debug` and some config file isn't
+    // trusted, Mercurial prints out a warning to stdout, twice, after Feb 2011.
+    //
+    // http://selenic.com/pipermail/mercurial-devel/2011-February/028541.html
+    //
+    // After Jan 2015, it may also fail to write to a revision branch cache.
+
+    $ignore = array(
+      'ignoring untrusted configuration option',
+      "couldn't write revision branch cache:",
+    );
+
+    foreach ($ignore as $key => $pattern) {
+      $ignore[$key] = preg_quote($pattern, '/');
+    }
+
+    $ignore = '('.implode('|', $ignore).')';
+
+    $lines = preg_split('/(?<=\n)/', $stdout);
+    $regex = '/'.$ignore.'.*\n$/';
+
+    foreach ($lines as $key => $line) {
+      $lines[$key] = preg_replace($regex, '', $line);
+    }
+
+    return implode('', $lines);
+  }
+
+}
diff --git a/src/applications/diffusion/protocol/DiffusionSubversionCommandEngine.php b/src/applications/diffusion/protocol/DiffusionSubversionCommandEngine.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/protocol/DiffusionSubversionCommandEngine.php
@@ -0,0 +1,54 @@
+<?php
+
+final class DiffusionSubversionCommandEngine
+  extends DiffusionCommandEngine {
+
+  protected function canBuildForRepository(
+    PhabricatorRepository $repository) {
+    return $repository->isSVN();
+  }
+
+  protected function newFormattedCommand($pattern, array $argv) {
+    $flags = array();
+    $args = array();
+
+    $flags[] = '--non-interactive';
+
+    if ($this->isAnyHTTPProtocol() || $this->isSVNProtocol()) {
+      $flags[] = '--no-auth-cache';
+
+      if ($this->isAnyHTTPProtocol()) {
+        $flags[] = '--trust-server-cert';
+      }
+
+      $credential_phid = $this->getCredentialPHID();
+      if ($credential_phid) {
+        $key = PassphrasePasswordKey::loadFromPHID(
+          $credential_phid,
+          PhabricatorUser::getOmnipotentUser());
+
+        $flags[] = '--username %P';
+        $args[] = $key->getUsernameEnvelope();
+
+        $flags[] = '--password %P';
+        $args[] = $key->getPasswordEnvelope();
+      }
+    }
+
+    $flags = implode(' ', $flags);
+    $pattern = "svn {$flags} {$pattern}";
+
+    return array($pattern, array_merge($args, $argv));
+  }
+
+  protected function newCustomEnvironment() {
+    $env = array();
+
+    if ($this->isAnySSHProtocol()) {
+      $env['SVN_SSH'] = $this->getSSHWrapper();
+    }
+
+    return $env;
+  }
+
+}
diff --git a/src/applications/diffusion/protocol/__tests__/DiffusionCommandEngineTestCase.php b/src/applications/diffusion/protocol/__tests__/DiffusionCommandEngineTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/applications/diffusion/protocol/__tests__/DiffusionCommandEngineTestCase.php
@@ -0,0 +1,155 @@
+<?php
+
+final class DiffusionCommandEngineTestCase extends PhabricatorTestCase {
+
+  public function testCommandEngine() {
+    $type_git = PhabricatorRepositoryType::REPOSITORY_TYPE_GIT;
+    $type_hg = PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL;
+    $type_svn = PhabricatorRepositoryType::REPOSITORY_TYPE_SVN;
+
+    $root = dirname(phutil_get_library_root('phabricator'));
+    $ssh_wrapper = $root.'/bin/ssh-connect';
+    $home = $root.'/support/empty/';
+
+
+    // Plain commands.
+
+    $this->assertCommandEngineFormat(
+      'git xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'HOME' => $home,
+      ),
+      array(
+        'vcs' => $type_git,
+        'argv' => 'xyz',
+      ));
+
+    $this->assertCommandEngineFormat(
+      'hg xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'HGPLAIN' => '1',
+      ),
+      array(
+        'vcs' => $type_hg,
+        'argv' => 'xyz',
+      ));
+
+    $this->assertCommandEngineFormat(
+      'svn --non-interactive xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+      ),
+      array(
+        'vcs' => $type_svn,
+        'argv' => 'xyz',
+      ));
+
+
+    // Commands with SSH.
+
+    $this->assertCommandEngineFormat(
+      'git xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'HOME' => $home,
+        'GIT_SSH' => $ssh_wrapper,
+      ),
+      array(
+        'vcs' => $type_git,
+        'argv' => 'xyz',
+        'protocol' => 'ssh',
+      ));
+
+    $this->assertCommandEngineFormat(
+      (string)csprintf('hg --config ui.ssh=%s xyz', $ssh_wrapper),
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'HGPLAIN' => '1',
+      ),
+      array(
+        'vcs' => $type_hg,
+        'argv' => 'xyz',
+        'protocol' => 'ssh',
+      ));
+
+    $this->assertCommandEngineFormat(
+      'svn --non-interactive xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'SVN_SSH' => $ssh_wrapper,
+      ),
+      array(
+        'vcs' => $type_svn,
+        'argv' => 'xyz',
+        'protocol' => 'ssh',
+      ));
+
+
+    // Commands with HTTP.
+
+    $this->assertCommandEngineFormat(
+      'git xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'HOME' => $home,
+      ),
+      array(
+        'vcs' => $type_git,
+        'argv' => 'xyz',
+        'protocol' => 'https',
+      ));
+
+    $this->assertCommandEngineFormat(
+      'hg xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+        'HGPLAIN' => '1',
+      ),
+      array(
+        'vcs' => $type_hg,
+        'argv' => 'xyz',
+        'protocol' => 'https',
+      ));
+
+    $this->assertCommandEngineFormat(
+      'svn --non-interactive --no-auth-cache --trust-server-cert xyz',
+      array(
+        'LANG' => 'en_US.UTF-8',
+      ),
+      array(
+        'vcs' => $type_svn,
+        'argv' => 'xyz',
+        'protocol' => 'https',
+      ));
+  }
+
+  private function assertCommandEngineFormat(
+    $command,
+    array $env,
+    array $inputs) {
+
+    $repository = id(new PhabricatorRepository())
+      ->setVersionControlSystem($inputs['vcs']);
+
+    $future = DiffusionCommandEngine::newCommandEngine($repository)
+      ->setArgv((array)$inputs['argv'])
+      ->setProtocol(idx($inputs, 'protocol'))
+      ->newFuture();
+
+    $command_string = $future->getCommand();
+
+    $actual_command = $command_string->getUnmaskedString();
+    $this->assertEqual($command, $actual_command);
+
+    $actual_environment = $future->getEnv();
+
+    $compare_environment = array_select_keys(
+      $actual_environment,
+      array_keys($env));
+
+    $this->assertEqual($env, $compare_environment);
+  }
+
+}
diff --git a/src/applications/diffusion/query/lowlevel/DiffusionLowLevelParentsQuery.php b/src/applications/diffusion/query/lowlevel/DiffusionLowLevelParentsQuery.php
--- a/src/applications/diffusion/query/lowlevel/DiffusionLowLevelParentsQuery.php
+++ b/src/applications/diffusion/query/lowlevel/DiffusionLowLevelParentsQuery.php
@@ -50,7 +50,9 @@
     list($stdout) = $repository->execxLocalCommand(
       'log --debug --limit 1 --template={parents} --rev %s',
       $this->identifier);
-    $stdout = PhabricatorRepository::filterMercurialDebugOutput($stdout);
+
+    $stdout = DiffusionMercurialCommandEngine::filterMercurialDebugOutput(
+      $stdout);
 
     $hashes = preg_split('/\s+/', trim($stdout));
     foreach ($hashes as $key => $value) {
diff --git a/src/applications/repository/storage/PhabricatorRepository.php b/src/applications/repository/storage/PhabricatorRepository.php
--- a/src/applications/repository/storage/PhabricatorRepository.php
+++ b/src/applications/repository/storage/PhabricatorRepository.php
@@ -487,19 +487,22 @@
   }
 
   private function newRemoteCommandFuture(array $argv) {
-    $argv = $this->formatRemoteCommand($argv);
-    $future = newv('ExecFuture', $argv);
-    $future->setEnv($this->getRemoteCommandEnvironment());
-    return $future;
+    return $this->newRemoteCommandEngine($argv)
+      ->newFuture();
   }
 
   private function newRemoteCommandPassthru(array $argv) {
-    $argv = $this->formatRemoteCommand($argv);
-    $passthru = newv('PhutilExecPassthru', $argv);
-    $passthru->setEnv($this->getRemoteCommandEnvironment());
-    return $passthru;
+    return $this->newRemoteCommandEngine($argv)
+      ->setPassthru(true)
+      ->newFuture();
   }
 
+  private function newRemoteCommandEngine(array $argv) {
+    return DiffusionCommandEngine::newCommandEngine($this)
+      ->setArgv($argv)
+      ->setCredentialPHID($this->getCredentialPHID())
+      ->setProtocol($this->getRemoteProtocol());
+  }
 
 /* -(  Local Command Execution  )-------------------------------------------- */
 
@@ -527,9 +530,9 @@
   private function newLocalCommandFuture(array $argv) {
     $this->assertLocalExists();
 
-    $argv = $this->formatLocalCommand($argv);
-    $future = newv('ExecFuture', $argv);
-    $future->setEnv($this->getLocalCommandEnvironment());
+    $future = DiffusionCommandEngine::newCommandEngine($this)
+      ->setArgv($argv)
+      ->newFuture();
 
     if ($this->usesLocalWorkingCopy()) {
       $future->setCWD($this->getLocalPath());
@@ -541,9 +544,10 @@
   private function newLocalCommandPassthru(array $argv) {
     $this->assertLocalExists();
 
-    $argv = $this->formatLocalCommand($argv);
-    $future = newv('PhutilExecPassthru', $argv);
-    $future->setEnv($this->getLocalCommandEnvironment());
+    $future = DiffusionCommandEngine::newCommandEngine($this)
+      ->setArgv($argv)
+      ->setPassthru(true)
+      ->newFuture();
 
     if ($this->usesLocalWorkingCopy()) {
       $future->setCWD($this->getLocalPath());
@@ -552,199 +556,6 @@
     return $future;
   }
 
-
-/* -(  Command Infrastructure  )--------------------------------------------- */
-
-
-  private function getSSHWrapper() {
-    $root = dirname(phutil_get_library_root('phabricator'));
-    return $root.'/bin/ssh-connect';
-  }
-
-  private function getCommonCommandEnvironment() {
-    $env = array(
-      // NOTE: Force the language to "en_US.UTF-8", which overrides locale
-      // settings. This makes stuff print in English instead of, e.g., French,
-      // so we can parse the output of some commands, error messages, etc.
-      'LANG' => 'en_US.UTF-8',
-
-      // Propagate PHABRICATOR_ENV explicitly. For discussion, see T4155.
-      'PHABRICATOR_ENV' => PhabricatorEnv::getSelectedEnvironmentName(),
-    );
-
-    switch ($this->getVersionControlSystem()) {
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
-        break;
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
-        // NOTE: See T2965. Some time after Git 1.7.5.4, Git started fataling if
-        // it can not read $HOME. For many users, $HOME points at /root (this
-        // seems to be a default result of Apache setup). Instead, explicitly
-        // point $HOME at a readable, empty directory so that Git looks for the
-        // config file it's after, fails to locate it, and moves on. This is
-        // really silly, but seems like the least damaging approach to
-        // mitigating the issue.
-
-        $root = dirname(phutil_get_library_root('phabricator'));
-        $env['HOME'] = $root.'/support/empty/';
-        break;
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
-        // NOTE: This overrides certain configuration, extensions, and settings
-        // which make Mercurial commands do random unusual things.
-        $env['HGPLAIN'] = 1;
-        break;
-      default:
-        throw new Exception(pht('Unrecognized version control system.'));
-    }
-
-    return $env;
-  }
-
-  private function getLocalCommandEnvironment() {
-    return $this->getCommonCommandEnvironment();
-  }
-
-  private function getRemoteCommandEnvironment() {
-    $env = $this->getCommonCommandEnvironment();
-
-    if ($this->shouldUseSSH()) {
-      // NOTE: This is read by `bin/ssh-connect`, and tells it which credentials
-      // to use.
-      $env['PHABRICATOR_CREDENTIAL'] = $this->getCredentialPHID();
-      switch ($this->getVersionControlSystem()) {
-        case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
-          // Force SVN to use `bin/ssh-connect`.
-          $env['SVN_SSH'] = $this->getSSHWrapper();
-          break;
-        case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
-          // Force Git to use `bin/ssh-connect`.
-          $env['GIT_SSH'] = $this->getSSHWrapper();
-          break;
-        case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
-          // We force Mercurial through `bin/ssh-connect` too, but it uses a
-          // command-line flag instead of an environmental variable.
-          break;
-        default:
-          throw new Exception(pht('Unrecognized version control system.'));
-      }
-    }
-
-    return $env;
-  }
-
-  private function formatRemoteCommand(array $args) {
-    $pattern = $args[0];
-    $args = array_slice($args, 1);
-
-    switch ($this->getVersionControlSystem()) {
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
-        if ($this->shouldUseHTTP() || $this->shouldUseSVNProtocol()) {
-          $flags = array();
-          $flag_args = array();
-          $flags[] = '--non-interactive';
-          $flags[] = '--no-auth-cache';
-          if ($this->shouldUseHTTP()) {
-            $flags[] = '--trust-server-cert';
-          }
-
-          $credential_phid = $this->getCredentialPHID();
-          if ($credential_phid) {
-            $key = PassphrasePasswordKey::loadFromPHID(
-              $credential_phid,
-              PhabricatorUser::getOmnipotentUser());
-            $flags[] = '--username %P';
-            $flags[] = '--password %P';
-            $flag_args[] = $key->getUsernameEnvelope();
-            $flag_args[] = $key->getPasswordEnvelope();
-          }
-
-          $flags = implode(' ', $flags);
-          $pattern = "svn {$flags} {$pattern}";
-          $args = array_mergev(array($flag_args, $args));
-        } else {
-          $pattern = "svn --non-interactive {$pattern}";
-        }
-        break;
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
-        $pattern = "git {$pattern}";
-        break;
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
-        if ($this->shouldUseSSH()) {
-          $pattern = "hg --config ui.ssh=%s {$pattern}";
-          array_unshift(
-            $args,
-            $this->getSSHWrapper());
-        } else {
-          $pattern = "hg {$pattern}";
-        }
-        break;
-      default:
-        throw new Exception(pht('Unrecognized version control system.'));
-    }
-
-    array_unshift($args, $pattern);
-
-    return $args;
-  }
-
-  private function formatLocalCommand(array $args) {
-    $pattern = $args[0];
-    $args = array_slice($args, 1);
-
-    switch ($this->getVersionControlSystem()) {
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
-        $pattern = "svn --non-interactive {$pattern}";
-        break;
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
-        $pattern = "git {$pattern}";
-        break;
-      case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
-        $pattern = "hg {$pattern}";
-        break;
-      default:
-        throw new Exception(pht('Unrecognized version control system.'));
-    }
-
-    array_unshift($args, $pattern);
-
-    return $args;
-  }
-
-  /**
-   * Sanitize output of an `hg` command invoked with the `--debug` flag to make
-   * it usable.
-   *
-   * @param string Output from `hg --debug ...`
-   * @return string Usable output.
-   */
-  public static function filterMercurialDebugOutput($stdout) {
-    // When hg commands are run with `--debug` and some config file isn't
-    // trusted, Mercurial prints out a warning to stdout, twice, after Feb 2011.
-    //
-    // http://selenic.com/pipermail/mercurial-devel/2011-February/028541.html
-    //
-    // After Jan 2015, it may also fail to write to a revision branch cache.
-
-    $ignore = array(
-      'ignoring untrusted configuration option',
-      "couldn't write revision branch cache:",
-    );
-
-    foreach ($ignore as $key => $pattern) {
-      $ignore[$key] = preg_quote($pattern, '/');
-    }
-
-    $ignore = '('.implode('|', $ignore).')';
-
-    $lines = preg_split('/(?<=\n)/', $stdout);
-    $regex = '/'.$ignore.'.*\n$/';
-
-    foreach ($lines as $key => $line) {
-      $lines[$key] = preg_replace($regex, '', $line);
-    }
-
-    return implode('', $lines);
-  }
-
   public function getURI() {
     $callsign = $this->getCallsign();
     if (strlen($callsign)) {
diff --git a/src/applications/repository/storage/__tests__/PhabricatorRepositoryTestCase.php b/src/applications/repository/storage/__tests__/PhabricatorRepositoryTestCase.php
--- a/src/applications/repository/storage/__tests__/PhabricatorRepositoryTestCase.php
+++ b/src/applications/repository/storage/__tests__/PhabricatorRepositoryTestCase.php
@@ -147,7 +147,8 @@
     );
 
     foreach ($map as $input => $expect) {
-      $actual = PhabricatorRepository::filterMercurialDebugOutput($input);
+      $actual = DiffusionMercurialCommandEngine::filterMercurialDebugOutput(
+        $input);
       $this->assertEqual($expect, $actual, $input);
     }
   }