Fix an issue with viewing an undiscovered commit in Diffusion

Summary: If you load Diffusion between a repository being pulled and discovered, you can end up with a valid commit reference that hasn't been discovered yet. Don't fatal.

Test Plan: Saw somewhat-helpful error page instead of fatal.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7448

Author: epriestley

src/applications/diffusion/request/DiffusionRequest.php

@@ -277,7 +277,9 @@ public function loadCommit() {
         'repositoryID = %d AND commitIdentifier = %s',
         $repository->getID(),
         $this->getCommit());
-      $commit->attachRepository($repository);
+      if ($commit) {
+        $commit->attachRepository($repository);
+      }
       $this->repositoryCommit = $commit;
     }
     return $this->repositoryCommit;