Show less misleading summary, test plan authors

Summary:
Instead of assuming the test plan and summary are written by the author
of the differential revision, let's assume they are written by the author
of the latest differential diff.

Test Plan: viewed a drev that had been commandeered but not updated to check authors

Reviewers: epriestley, jungejason, vrana

Reviewed By: epriestley

CC: aran, Koolvin

Maniphest Tasks: T1235

Differential Revision: https://secure.phabricator.com/D2550