Mark git-hook-pre-receive as git only

Summary: It doesn't make any sense to run this command on another VCS.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11624

src/workflow/ArcanistGitHookPreReceiveWorkflow.php

@@ -41,6 +41,10 @@ EOTEXT
     return false;
   }
 
+  public function getSupportedRevisionControlSystems() {
+    return array('git');
+  }
+
   public function run() {
     $working_copy = $this->getWorkingCopy();
     if (!$working_copy->getProjectID()) {