Correct the generation of "<th />" IDs on left-hand-side of image changesets

Summary:
Fixes T7682. The left-hand-side "<th />" row did not generate with the correct ID.

(I couldn't reproduce the exact issue described in T7682, but hovering comments on either side now works properly for me.)

Test Plan: {F4962479}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7682

Differential Revision: https://secure.phabricator.com/D17926

src/applications/differential/render/DifferentialChangesetTwoUpRenderer.php

@@ -323,6 +323,12 @@ final class DifferentialChangesetTwoUpRenderer
       $new = $this->renderImageStage($new_file);
     }
 
+    // If we don't have an explicit "vs" changeset, it's the left side of the
+    // "id" changeset.
+    if (!$vs) {
+      $vs = $id;
+    }
+
     $html_old = array();
     $html_new = array();
     foreach ($this->getOldComments() as $on_line => $comment_group) {