Add needActiveDiffs to differential.createcomment method

Summary: Ref T12766. Adds missing attachment for stacking actions in differential

Test Plan: Asked end user to verify patch.

Reviewers: epriestley, amckinley

Reviewed By: epriestley, amckinley

Subscribers: reed, Korvin

Maniphest Tasks: T12766

Differential Revision: https://secure.phabricator.com/D18038

src/applications/differential/conduit/DifferentialCreateCommentConduitAPIMethod.php

@@ -49,6 +49,7 @@ final class DifferentialCreateCommentConduitAPIMethod
       ->withIDs(array($request->getValue('revision_id')))
       ->needReviewers(true)
       ->needReviewerAuthority(true)
+      ->needActiveDiffs(true)
       ->executeOne();
     if (!$revision) {
       throw new ConduitException('ERR_BAD_REVISION');