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 master, got just master.
• Called the API method with maste*, got master. This behavior is not officially supported and may change in the future.
• Viewed a commit, still saw all branches.
  • Grepped for diffusion.branchquery and verified that no remaining callsites pass a default "branch" parameter.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19499