Maybe fix excluding to / cc folks on differential emails

Summary: Ref T5185. By code inspection, I am pretty sure before this patch it was doing a set of a get on itself which does nothing. Now, being careful not to break Facebook we get the proper exclusion phids. I am pretty sure the folks in T5185 are experiencing this in Differential only.

Test Plan: Get some folks on T5185 to play with this

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5185

Differential Revision: https://secure.phabricator.com/D10087