Fix undefinded $changesets when submitting inline comments in Differential

Summary: Fixes T5262. This branch is overzealous, and causes us to fail to load changeses if `metamta.differential.unified-comment-context` is off. It was on for me locally for testing, which is why I missed this.

Test Plan: No more exception.

Reviewers: chad

Reviewed By: chad

Subscribers: richardvanvelzen, epriestley

Maniphest Tasks: T5262

Differential Revision: https://secure.phabricator.com/D9376

src/applications/differential/editor/DifferentialTransactionEditor.php

@@ -1284,19 +1284,20 @@ final class DifferentialTransactionEditor
       $changeset_ids[$id] = $id;
     }
 
-    if ($show_context) {
-      $hunk_parser = new DifferentialHunkParser();
     $changesets = id(new DifferentialChangesetQuery())
       ->setViewer($this->getActor())
       ->withIDs($changeset_ids)
       ->needHunks(true)
       ->execute();
-    }
 
     $inline_groups = DifferentialTransactionComment::sortAndGroupInlines(
       $inlines,
       $changesets);
 
+    if ($show_context) {
+      $hunk_parser = new DifferentialHunkParser();
+    }
+
     $result = array();
     foreach ($inline_groups as $changeset_id => $group) {
       $changeset = idx($changesets, $changeset_id);