Add basic support for mirroring Mercurial repositories

Summary:
Ref T4338. Mercurial exits with exit code 1 and "no changes found" in stdout
when there's no changes. I've split up the `pushRepositoryToMirror` to make
the code a tad more readable.

It isn't perfect, but it works for me.

Test Plan:
pushed some changes to my hosted repo. Saw them appearing in the
mirrored repo

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4338

Differential Revision: https://secure.phabricator.com/D8107

src/applications/repository/engine/PhabricatorRepositoryMirrorEngine.php

@@ -55,9 +55,17 @@ final class PhabricatorRepositoryMirrorEngine
 
     $this->log(pht('Pushing to remote "%s"...', $mirror->getRemoteURI()));
 
-    if (!$proxy->isGit()) {
+    if ($proxy->isGit()) {
+      $this->pushToGitRepository($proxy);
+    } else if ($proxy->isHg()) {
+      $this->pushToHgRepository($proxy);
+    } else {
       throw new Exception(pht('Unsupported VCS!'));
     }
+  }
+
+  private function pushToGitRepository(
+    PhabricatorRepository $proxy) {
 
     $future = $proxy->getRemoteCommandFuture(
       'push --verbose --mirror -- %P',
@@ -68,4 +76,24 @@ final class PhabricatorRepositoryMirrorEngine
       ->resolvex();
   }
 
+  private function pushToHgRepository(
+    PhabricatorRepository $proxy) {
+
+    $future = $proxy->getRemoteCommandFuture(
+      'push --verbose --rev tip -- %P',
+      $proxy->getRemoteURIEnvelope());
+
+    try {
+      $future
+        ->setCWD($proxy->getLocalPath())
+        ->resolvex();
+    } catch (CommandException $ex) {
+      if (preg_match('/no changes found/', $ex->getStdOut())) {
+        // mercurial says nothing changed, but that's good
+      } else {
+        throw $ex;
+      }
+    }
+  }
+
 }

src/applications/repository/storage/PhabricatorRepository.php

@@ -1038,7 +1038,7 @@ final class PhabricatorRepository extends PhabricatorRepositoryDAO
   }
 
   public function canMirror() {
-    if ($this->isGit()) {
+    if ($this->isGit() || $this->isHg()) {
       return true;
     }