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