Use first diff author for summary and test plan in commandeered revisions

Summary:
I thought about it a little bit and this makes the most sense for me:

1. Original author usually writes at least something and commander only updates it.
2. There's a creation date of revision (= first diff) by these comments. I don't want to change this date because I use this information. Author should correspond to this date.
3. It solves all our repros.

Test Plan: Display commandeered revision.

Reviewers: nh, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2577