Support automatic "Depends On" detection in Mercurial

Summary: Depends on D18651. Allows Mercurial to auto-detect depends-on revisions.

Test Plan: See D18652 and D18653. Didn't actually test the empty repository states but they're probably okay. The `experimental` branch is an untamed wild. ¯\_(ツ)_/¯

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18654

src/repository/api/ArcanistGitAPI.php

@@ -1054,7 +1054,12 @@ final class ArcanistGitAPI extends ArcanistRepositoryAPI {
   }
 
   public function getBaseCommitRef() {
-    $base_commit = $this->getSourceControlBaseRevision();
+    $base_commit = $this->getBaseCommit();
+
+    if ($base_commit === self::GIT_MAGIC_ROOT_COMMIT) {
+      return null;
+    }
+
     $base_message = $this->getCommitMessage($base_commit);
 
     // TODO: We should also pull the tree hash.

src/repository/api/ArcanistMercurialAPI.php

@@ -596,6 +596,20 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
     return $refs;
   }
 
+  public function getBaseCommitRef() {
+    $base_commit = $this->getBaseCommit();
+
+    if ($base_commit === 'null') {
+      return null;
+    }
+
+    $base_message = $this->getCommitMessage($base_commit);
+
+    return $this->newCommitRef()
+      ->setCommitHash($base_commit)
+      ->attachMessage($base_message);
+  }
+
   public function hasLocalCommit($commit) {
     try {
       $this->getCanonicalRevisionName($commit);