Don't interpret arc browse . as a commit
Summary: Ref T5781. git show . works like HEAD, but that isn't what arc browse . means.
Test Plan: Ran arc browse . with a repository at a published commit.
Reviewers: chad, btrahan, avive, avivey
Reviewed By: avivey
Subscribers: epriestley, avivey
Maniphest Tasks: T5781
Differential Revision: https://secure.phabricator.com/D10197