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 several repositories.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2716

Differential Revision: https://secure.phabricator.com/D7451