Fix issue in Mercurial repos with duplicate branch heads

Summary:
Fixes T5613. A branch may have multiple heads in Mercurial, but `executeOne()` expects exactly one result.

Load them all instead. Equivalently, we could `limit(1)`, but it's likely that we'll use the cursors in the future to reduce the number of VCS operations we do, so this is probably a little more along the lines where we're headed.

Test Plan: Poked around some repos.

Reviewers: chad, richardvanvelzen

Reviewed By: richardvanvelzen

Subscribers: epriestley

Maniphest Tasks: T5613

Differential Revision: https://secure.phabricator.com/D9918

src/applications/diffusion/controller/DiffusionRepositoryController.php

@@ -29,13 +29,14 @@ final class DiffusionRepositoryController extends DiffusionController {
     // If this VCS supports branches, check that the selected branch actually
     // exists.
     if ($drequest->supportsBranches()) {
-      $ref_cursor = id(new PhabricatorRepositoryRefCursorQuery())
+      // NOTE: Mercurial may have multiple branch heads with the same name.
+      $ref_cursors = id(new PhabricatorRepositoryRefCursorQuery())
         ->setViewer($viewer)
         ->withRepositoryPHIDs(array($repository->getPHID()))
         ->withRefTypes(array(PhabricatorRepositoryRefCursor::TYPE_BRANCH))
         ->withRefNames(array($drequest->getBranch()))
-        ->executeOne();
-      if ($ref_cursor) {
+        ->execute();
+      if ($ref_cursors) {
         // This is a valid branch, so we necessarily have some content.
         $page_has_content = true;
       } else {