Add a branch selector to Diffusion

Summary: Fixes T12931. Adds a branch selector that's always visible if the repo has commits.

Test Plan:
Test a plain hg, svn, git repository. Test setting a bad default branch. Test a good default branch. Test on desktop, mobile layouts.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12931

Differential Revision: https://secure.phabricator.com/D18267