Ignore detached git branches.

Summary:
Fixes T4559.

It looks like the code currently (at least partially) handles this by checking for (no branch). I suspect that the behaviour of git has changed (I am running version 1.9.0) because I haven't figured out what state to be in to cause git to output (no branch).

Test Plan: Ran arc branch when on a detached branch.

Reviewers: Blessed Reviewers, epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4559

Differential Revision: https://secure.phabricator.com/D8466