Show refs in Diffusion for Git commits

Summary: In Diffusion, for Git, show commit refs (like "origin/master, origin/HEAD").

Test Plan: Looked at several Git, Hg and SVN commits.

Reviewers: davidreuss, btrahan, vrana, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1130

Differential Revision: https://secure.phabricator.com/D2311