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