Merge diffusion.commitbranchesquery into diffusion.branchquery

Summary:
Ref T4327. This is general cleanup since I was in this area of the code. Primarily, the Mercurial implementation here was completely broken and wrong:

• It returned only one branch, but a commit can be present on many branches.
• It did not account for multiple branch heads.
• It returned a result implying the branch head pointed at the queried commit, which is no consistent or accurate.

Simplify the amount of API we're dealing with by collapsing this method into the very similar diffusion.branchquery method.

Test Plan: Looked at mercurial and git repositories and commits, branch information seemed correct.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D8003