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