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
