We had a very, very old bug with post-commit review where the diffs from
the posted commit would be attached to the Review Request's diffset
history, rather than the draft. This made the diff accessible to
superusers who had access to the review request, which wasn't a big
deal, but more importantly it meant the diff wasn't accessible via the
draft API and default reviewers would not apply.

This change fixes this by redoing some of that logic. Formerly, the code
for updating was assuming it would be called on either the draft or
review request, but in practice now it's only ever called on the draft,
so that has moved into ReviewRequestDraft and has been updated to
associate there instead of the review request history.

That also required some new code that figured out a suitable revision
for the diffset. Since that's already been done in two other places, the
logic has been centralized (and optimized to reduce the data returned
from an SQL query).

Some unit tests were added and others moved around to test against the
draft instead of review request.

Unit tests pass.
Tested post-commit review with default reviewers. It worked.