Allow "arc browse <symbol>" to resolve symbolic commits

Summary: Ref T5781. This makes things like `arc browse master` work (but they open the commit, not a revision).

Test Plan: Ran `arc browse master`.

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781

Differential Revision: https://secure.phabricator.com/D10143