diff --git a/src/repository/api/base/ArcanistRepositoryAPI.php b/src/repository/api/base/ArcanistRepositoryAPI.php
index 1a987707..b3031ce1 100644
--- a/src/repository/api/base/ArcanistRepositoryAPI.php
+++ b/src/repository/api/base/ArcanistRepositoryAPI.php
@@ -159,4 +159,15 @@ abstract class ArcanistRepositoryAPI {
   abstract public function getCurrentFileData($path);
   abstract public function getLocalCommitInformation();
 
+
+  abstract public function supportsRelativeLocalCommits();
+
+  public function parseRelativeLocalCommit(array $argv) {
+    throw new Exception("This VCS does not support relative local commits.");
+  }
+
+  public function getAllLocalChanges() {
+    throw new Exception("This VCS does not support getting all local changes.");
+  }
+
 }
diff --git a/src/repository/api/git/ArcanistGitAPI.php b/src/repository/api/git/ArcanistGitAPI.php
index 17181d76..db16cd87 100644
--- a/src/repository/api/git/ArcanistGitAPI.php
+++ b/src/repository/api/git/ArcanistGitAPI.php
@@ -512,4 +512,37 @@ class ArcanistGitAPI extends ArcanistRepositoryAPI {
     return trim($owner);
   }
 
+  public function supportsRelativeLocalCommits() {
+    return true;
+  }
+
+  public function parseRelativeLocalCommit(array $argv) {
+    if (count($argv) == 0) {
+      return;
+    }
+    if (count($argv) != 1) {
+      throw new ArcanistUsageException("Specify only one commit.");
+    }
+    $base = reset($argv);
+    if ($base == ArcanistGitAPI::GIT_MAGIC_ROOT_COMMIT) {
+      $merge_base = $base;
+    } else {
+      list($err, $merge_base) = exec_manual(
+        '(cd %s; git merge-base %s HEAD)',
+        $this->getPath(),
+        $base);
+      if ($err) {
+        throw new ArcanistUsageException(
+          "Unable to parse git commit name '{$base}'.");
+      }
+    }
+    $this->setRelativeCommit(trim($merge_base));
+  }
+
+  public function getAllLocalChanges() {
+    $diff = $this->getFullGitDiff();
+    $parser = new ArcanistDiffParser();
+    return $parser->parseDiff($diff);
+  }
+
 }
diff --git a/src/repository/api/git/__init__.php b/src/repository/api/git/__init__.php
index c225acc7..a7fd205d 100644
--- a/src/repository/api/git/__init__.php
+++ b/src/repository/api/git/__init__.php
@@ -6,6 +6,8 @@
 
 
 
+phutil_require_module('arcanist', 'exception/usage');
+phutil_require_module('arcanist', 'parser/diff');
 phutil_require_module('arcanist', 'repository/api/base');
 
 phutil_require_module('phutil', 'future');
diff --git a/src/repository/api/mercurial/ArcanistMercurialAPI.php b/src/repository/api/mercurial/ArcanistMercurialAPI.php
index db6fbfa4..db78f0b5 100644
--- a/src/repository/api/mercurial/ArcanistMercurialAPI.php
+++ b/src/repository/api/mercurial/ArcanistMercurialAPI.php
@@ -374,4 +374,26 @@ class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     return trim($match[1]);
   }
 
+  public function supportsRelativeLocalCommits() {
+    return true;
+  }
+
+  public function parseRelativeLocalCommit(array $argv) {
+    if (count($argv) == 0) {
+      return;
+    }
+    if (count($argv) != 1) {
+      throw new ArcanistUsageException("Specify only one commit.");
+    }
+    // This does the "hg id" call we need to normalize/validate the revision
+    // identifier.
+    $this->setRelativeCommit(reset($argv));
+  }
+
+  public function getAllLocalChanges() {
+    $diff = $this->getFullMercurialDiff();
+    $parser = new ArcanistDiffParser();
+    return $parser->parseDiff($diff);
+  }
+
 }
diff --git a/src/repository/api/subversion/ArcanistSubversionAPI.php b/src/repository/api/subversion/ArcanistSubversionAPI.php
index 4616ed35..a9179803 100644
--- a/src/repository/api/subversion/ArcanistSubversionAPI.php
+++ b/src/repository/api/subversion/ArcanistSubversionAPI.php
@@ -482,4 +482,8 @@ EODIFF;
     return null;
   }
 
+  public function supportsRelativeCommits() {
+    return false;
+  }
+
 }
diff --git a/src/workflow/base/ArcanistBaseWorkflow.php b/src/workflow/base/ArcanistBaseWorkflow.php
index 59e5cc8a..6a5a242b 100644
--- a/src/workflow/base/ArcanistBaseWorkflow.php
+++ b/src/workflow/base/ArcanistBaseWorkflow.php
@@ -768,6 +768,8 @@ class ArcanistBaseWorkflow {
     $repository_api = $this->getRepositoryAPI();
 
     if ($repository_api instanceof ArcanistSubversionAPI) {
+      // NOTE: In SVN, we don't currently support a "get all local changes"
+      // operation, so special case it.
       if (empty($this->changeCache[$path])) {
         $diff = $repository_api->getRawDiffText($path);
         $parser = new ArcanistDiffParser();
@@ -777,15 +779,15 @@ class ArcanistBaseWorkflow {
         }
         $this->changeCache[$path] = reset($changes);
       }
-    } else {
+    } else if ($repository_api->supportsRelativeLocalCommits()) {
       if (empty($this->changeCache)) {
-        $diff = $repository_api->getFullGitDiff();
-        $parser = new ArcanistDiffParser();
-        $changes = $parser->parseDiff($diff);
+        $changes = $repository_api->getAllLocalChanges();
         foreach ($changes as $change) {
           $this->changeCache[$change->getCurrentPath()] = $change;
         }
       }
+    } else {
+      throw new Exception("Missing VCS support.");
     }
 
     if (empty($this->changeCache[$path])) {
@@ -827,30 +829,6 @@ class ArcanistBaseWorkflow {
     }
   }
 
-  protected function parseGitRelativeCommit(ArcanistGitAPI $api, array $argv) {
-    if (count($argv) == 0) {
-      return;
-    }
-    if (count($argv) != 1) {
-      throw new ArcanistUsageException(
-        "Specify exactly one commit.");
-    }
-    $base = reset($argv);
-    if ($base == ArcanistGitAPI::GIT_MAGIC_ROOT_COMMIT) {
-      $merge_base = $base;
-    } else {
-      list($err, $merge_base) = exec_manual(
-        '(cd %s; git merge-base %s HEAD)',
-        $api->getPath(),
-        $base);
-      if ($err) {
-        throw new ArcanistUsageException(
-          "Unable to parse git commit name '{$base}'.");
-      }
-    }
-    $api->setRelativeCommit(trim($merge_base));
-  }
-
   protected function normalizeRevisionID($revision_id) {
     return ltrim(strtoupper($revision_id), 'D');
   }
diff --git a/src/workflow/base/__init__.php b/src/workflow/base/__init__.php
index a7a8889f..a596254a 100644
--- a/src/workflow/base/__init__.php
+++ b/src/workflow/base/__init__.php
@@ -13,7 +13,6 @@ phutil_require_module('arcanist', 'exception/usage/userabort');
 phutil_require_module('arcanist', 'parser/bundle');
 phutil_require_module('arcanist', 'parser/diff');
 phutil_require_module('arcanist', 'parser/diff/change');
-phutil_require_module('arcanist', 'repository/api/git');
 
 phutil_require_module('phutil', 'conduit/client');
 phutil_require_module('phutil', 'console');
diff --git a/src/workflow/diff/ArcanistDiffWorkflow.php b/src/workflow/diff/ArcanistDiffWorkflow.php
index 8fa93390..2a25f73a 100644
--- a/src/workflow/diff/ArcanistDiffWorkflow.php
+++ b/src/workflow/diff/ArcanistDiffWorkflow.php
@@ -603,17 +603,10 @@ EOTEXT
         }
       }
 
-    } else if ($repository_api instanceof ArcanistGitAPI) {
-      $this->parseGitRelativeCommit(
-        $repository_api,
+    } else if ($repository_api->supportsRelativeLocalCommits()) {
+      $repository_api->parseRelativeLocalCommit(
         $this->getArgument('paths', array()));
       $paths = $repository_api->getWorkingCopyStatus();
-    } else if ($repository_api instanceof ArcanistMercurialAPI) {
-      // TODO: Unify this and the previous block.
-
-      // TODO: Parse the relative commit.
-
-      $paths = $repository_api->getWorkingCopyStatus();
     } else {
       throw new Exception("Unknown VCS!");
     }
diff --git a/src/workflow/lint/ArcanistLintWorkflow.php b/src/workflow/lint/ArcanistLintWorkflow.php
index 938b8033..c98d6a26 100644
--- a/src/workflow/lint/ArcanistLintWorkflow.php
+++ b/src/workflow/lint/ArcanistLintWorkflow.php
@@ -125,11 +125,12 @@ EOTEXT
             unset($paths[$path]);
           }
         }
-      } else {
-        $this->parseGitRelativeCommit(
-          $repository_api,
-          $this->getArgument('paths'));
+      } else if ($repository_api->supportsRelativeLocalCommits()) {
+        $repository_api->parseRelativeLocalCommit(
+          $this->getArgument('paths', array()));
         $paths = $repository_api->getWorkingCopyStatus();
+      } else {
+        throw new Exception("Unknown VCS!");
       }
 
       foreach ($paths as $path => $flags) {
@@ -139,7 +140,6 @@ EOTEXT
       }
 
       $paths = array_keys($paths);
-
     } else {
       $paths = $this->getArgument('paths');
       if (empty($paths)) {