Support Mercurial pretxnchangegroup hooks

Summary: Ref T4189. Fixes T2066. Mercurial has a //lot// of hooks so I'm not 100% sure this is all we need to install (we may need separate hooks for tags/bookmarks) but it should cover most of what we're after at least.

Test Plan:
  - `bin/repository pull`'d a Mercurial repo and got a hook install.
  - Pushed to a Mercurial repository over SSH and HTTP, with good/bad hooks. Saw hooks fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2066, T4189

Differential Revision: https://secure.phabricator.com/D7685

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -51,6 +51,9 @@ final class DiffusionCommitHookEngine extends Phobject {
       case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
         $err = $this->executeSubversionHook();
         break;
+      case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
+        $err = $this->executeMercurialHook();
+        break;
       default:
         throw new Exception(pht('Unsupported repository type "%s"!', $type));
     }
@@ -73,6 +76,13 @@ final class DiffusionCommitHookEngine extends Phobject {
     return 0;
   }
 
+  private function executeMercurialHook() {
+
+    // TODO: Here, too, useful things should be done.
+
+    return 0;
+  }
+
   private function parseGitUpdates($stdin) {
     $updates = array();