Fix a commit hook issue where pushing dangerous changes would fatal before hitting the dragon bureaucrats

Summary: See <https://discourse.phabricator-community.org/t/php-fatal-when-using-git-push-d/1317>. The behavioral changes for Herald on initial import from D19265 could leave `$all_updates` undefined if we throw early enough.

Test Plan: Pushed a dangerous change, saw dragon bureaucrats again.

Differential Revision: https://secure.phabricator.com/D19297

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -126,6 +126,7 @@ final class DiffusionCommitHookEngine extends Phobject {
 
   public function execute() {
     $ref_updates = $this->findRefUpdates();
+    $all_updates = $ref_updates;
 
     $caught = null;
     try {