Fix Differential flag view bug

Summary: See D3125#3

Test Plan:
Well, apparently the FB test Phabricator has no other flags
in it, so I modified the database by hand... the changed revision was
not marked as flagged in Differential.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1557

Differential Revision: https://secure.phabricator.com/D3130