Fix ambiguous argument where a branch has the same name as a file

Summary: Currently, if you have a branch named "docs" and a local file named "docs", git show -s docs complains because it's ambiguous. Use -- to unambiguously mark branches as revisions, not files.

Test Plan: Ran arc branch in a working copy with a "docs" branch and a "docs" file, got expected results.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3030