R1:1377d349e1ad

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:

# Original author usually writes at least something and commander only updates it.
# 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.
# It solves all our repros.

Test Plan: Display commandeered revision.

Reviewers: nh, epriestley…