R1:8a0a00f118be

Make PhabricatorRepositoryCommmit schema changes for audit

Summary:
- Add a proper mailKey field to make these things mailable. Backfill all
existing objects.
- Denormalize authorPHID to the commit object so we can query by it
efficiently in a future diff. We currently use the search engine to drive
"commits by author" but that's not so good for audit, which needs more
constraints.
- Add an overall audit status field so we can efficiently query "commits that
needs your attention".
- Add enough…