Prevent using deleted repositories when displaying commits for a revision.

Summary:
We deleted a repository (I don't remember which one). Some
differential revisions where committed to this repository, and opening
these revisions show a blank page, because the repository_id is not in
the database anymore.

This causes the 'ERROR 8: Undefined index: 12 at
[phabricator/src/applications/phid/handle/data/PhabricatorObjectHandleData.php:209]
' in our log.

Test Plan:
Opened a revision which where committed to multiple repositories including
the deleted one. Made sure that the page was rendered correctly and there
were no messages in the error log.

Reviewed By: epriestley
Reviewers: epriestley, jungejason
CC: aran, epriestley
Differential Revision: 598

src/applications/phid/handle/data/PhabricatorObjectHandleData.php

@@ -202,7 +202,8 @@ class PhabricatorObjectHandleData {
             $handle = new PhabricatorObjectHandle();
             $handle->setPHID($phid);
             $handle->setType($type);
-            if (empty($commits[$phid])) {
+            if (empty($commits[$phid]) ||
+                !isset($callsigns[$repository_ids[$phid]])) {
               $handle->setName('Unknown Commit');
             } else {
               $commit = $commits[$phid];