Fix !unsubscribe in Differential

Summary: Fixes T5682.

Test Plan: Will test...

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5682

Differential Revision: https://secure.phabricator.com/D10013

src/applications/differential/mail/DifferentialReplyHandler.php

@@ -120,6 +120,7 @@ class DifferentialReplyHandler extends PhabricatorMailReplyHandler {
     switch ($command) {
       case 'unsubscribe':
         id(new PhabricatorSubscriptionsEditor())
+          ->setActor($actor)
           ->setObject($this->getMailReceiver())
           ->unsubscribe(array($actor->getPHID()))
           ->save();