Simplify Mercurial ref resolution; expose "closed" at top-level

Summary: Ref T7100. Ref T6160. Share branch code. Surface "closed".

Test Plan: Browsed a mercurial repository and saw consistent ref/cache state.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6160, T7100

Differential Revision: https://secure.phabricator.com/D12551