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

src/applications/diffusion/controller/DiffusionRepositoryController.php

@@ -351,6 +351,7 @@ final class DiffusionRepositoryController extends DiffusionController {
     $branches = $this->callConduitWithDiffusionRequest(
       'diffusion.branchquery',
       array(
+        'closed' => false,
         'limit' => $limit + 1,
       ));
     if (!$branches) {