Add indication of hg branch open/closed in branch list

Summary: Adds some basic UI for open / closed state when viewing a list of branches in Mercurial. Fixes T12838

Test Plan: Close and open branches, view list.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12838

Differential Revision: https://secure.phabricator.com/D18447