Fix an issue with rendering unit messages for diffs with no buildable

Summary: Fixes T10591. This was accidentally reverted in 148a50e48b, probably when resolvign a merge/rebase.

Test Plan: Will push to production.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10591

Differential Revision: https://secure.phabricator.com/D15474

src/applications/differential/controller/DifferentialRevisionViewController.php

@@ -1131,6 +1131,10 @@ final class DifferentialRevisionViewController extends DifferentialController {
     DifferentialRevision $revision) {
     $viewer = $this->getViewer();
 
+    if (!$diff->getBuildable()) {
+      return null;
+    }
+
     if (!$diff->getUnitMessages()) {
       return null;
     }