Fix an issue where Diffusion could fatal if the default branch was deleted

Summary: See PHI234. In T12931 we improved the behavior of Diffusion when a repository's default branch is set to a branch that does not exist, but in T11823 the way refcursors work changed, and we can now get a cursor (just with no positions) back for a deleted branch. When we did, we didn't handle things gracefully.

Test Plan:
  - Set default branch to a deleted branch, saw nice error instead of fatal.
  - Set default branch to a nonexistent branch which never existed, saw nice error.
  - Set default branch to existing "master", saw repository normally.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18811

src/applications/diffusion/controller/DiffusionRepositoryController.php

@@ -46,8 +46,20 @@ final class DiffusionRepositoryController extends DiffusionController {
         ->withRepositoryPHIDs(array($repository->getPHID()))
         ->withRefTypes(array(PhabricatorRepositoryRefCursor::TYPE_BRANCH))
         ->withRefNames(array($drequest->getBranch()))
+        ->needPositions(true)
         ->execute();
-      if ($ref_cursors) {
+
+      // It's possible that this branch previously existed, but has been
+      // deleted. Make sure we have valid cursor positions, not just cursors.
+      $any_positions = false;
+      foreach ($ref_cursors as $ref_cursor) {
+        if ($ref_cursor->getPositions()) {
+          $any_positions = true;
+          break;
+        }
+      }
+
+      if ($any_positions) {
         // This is a valid branch, so we necessarily have some content.
         $page_has_content = true;
       } else {