R1:8ab8c390b785

If "branch" is provided to "diffusion.branchquery", use it as the "<pattern>" argument to "git branch --contains ..."

Summary:
Ref T13151. See PHI720. If you want to test if commit X appears on specific branch Y, `git branch --contains X -- Y` is faster than (effectively) `git branch --contains X | grep Y`.

Since this call has a "branch" parameter anyway, use it as the pattern argument if provided.

Test Plan:
- Called the API method with no parameters, got all branches.
- Called the API method with…