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.
{F5058061}
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T12931
Differential Revision: https://secure.phabricator.com/D18267
69a7d57c3f