Track closed branches in Mercurial

Summary: Ref T6160. Ref T7100. Mercurial branch heads can be closed; track this state so we can be smarter about it.

Test Plan: Closed a branch, run repository update, saw it close in the cursor table.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6160, T7100

Differential Revision: https://secure.phabricator.com/D12550