Ignore closed branch heads by default in Mercurial

Summary:
Fixes T6160. Ref T7100.

• When resolving ambiguous branch references, ignore closed heads unless there are no other options.
• Hide closed heads by default on the main page.
• Show branch open/closed state in Mercurial.

Test Plan: Browsed a previously-ambiguous Mercurial repository because of multiple branch heads, no longer ambiguous.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6160, T7100

Differential Revision: https://secure.phabricator.com/D12552