R1:8b4b614d15f8

Show all branches a commit appears on

Summary:
We show the contextual branch (always the repository default branch) when
viewing a commit. Instead, show all branches the commit appears on.

Also pull some of the duplicated DiffusionXQuery stuff into a DiffusionQuery
base class, I'll do a followup to reduce more duplication.

Test Plan: Looked at a commit in Git. My HG and SVN setups are a little borked
so I kind of faked tests in them -- I'm fixing them now.

Reviewers: btrahan, jungejason, fratrik…