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

src/applications/diffusion/controller/DiffusionRefTableController.php

@@ -75,7 +75,7 @@ final class DiffusionRefTableController extends DiffusionController {
 
         $cached_hash = idx($cache, 'identifier');
         if ($cached_hash !== null) {
-          $cache_hash = DiffusionView::linkCommit(
+          $cached_hash = DiffusionView::linkCommit(
             $repository,
             $cached_hash);
         }