Properly exclude others' revisions from automatic "Depends On"

Summary: See D18651. This was missing an `unset()`.

Test Plan: Looked carefully.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18766

src/workflow/ArcanistDiffWorkflow.php

@@ -2961,6 +2961,7 @@ EOTEXT
 
       // Don't automatically depend on revisions authored by other users.
       if ($revision_ref->getAuthorPHID() != $viewer_phid) {
+        unset($revision_refs[$key]);
         continue;
       }
     }