Rewrite CommitQuery to use UNION for performance

Summary:
Ref T12680. See PHI167. See that task for discussion.

Rewrite DiffusionCommitQuery to work more like DifferentialRevisionQuery, and use a UNION to find "all revisions you need to audit OR respond to".

I tried to get this working a little more cleanly than RevisionQuery does, and can probably simplify that now.

Test Plan: Poked at the UI locally without hitting any apparent issues, but my local data is pretty garbage at this point. I'll take a look at how the query plans work on secure.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12680

Differential Revision: https://secure.phabricator.com/D18722