R1:35ccda922a31

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…