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