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