R1:d1c4b5081cd0

Clean up Diffusion branch query a bit

Summary:
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.

Test Plan: Looked at branches in…