Fix the "Browse in Diffusion" URI in Differential

Summary: Apparently I spent like a good month copy/pasting slightly different versions of this logic all over the codebase.

Test Plan: Selected "View Options -> Browse in Diffusion" for a chagneset, got a URI with a branch name in it under Git.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1993