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.