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