R1:d3436c256cba

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…