Fix a Diffusion issue where commits that do not show changesets would incorrectly try to render changesets

Summary:
See <https://discourse.phabricator-community.org/t/loading-certain-svn-commits-cause-unhandled-exception/3795/>.

Commits with no changesets (for example, deleted commits) don't generate a "$changesets".

Test Plan: Viewed a commit with no changesets. Before change: exception. After change: saw unusual commit state.

Differential Revision: https://secure.phabricator.com/D21175

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -459,9 +459,12 @@ final class DiffusionCommitController extends DiffusionController {
 
     $filetree = id(new DifferentialFileTreeEngine())
       ->setViewer($viewer)
-      ->setChangesets($changesets)
       ->setDisabled(!$show_changesets);
 
+    if ($show_changesets) {
+      $filetree->setChangesets($changesets);
+    }
+
     $description_box = id(new PHUIObjectBoxView())
       ->setHeaderText(pht('Description'))
       ->setBackground(PHUIObjectBoxView::BLUE_PROPERTY)