Ref T2716.
- Serve from DiffusionCommitQuery, not PhabricatorAuditCommitQuery (which should probably die).
- Fix logic for limit, which incorrectly failed to display the "Showing %d branches." text.
- Clean up things a touch.
- I didn't end up actually needing needCommitData(), but left it in there since I think it will be needed soon.
- Removed a "TODO" because I don't remember what "etc etc" means.