Possible fix for T1076, pushing to verify.

I think the issue is that we don't set the left-side changesetID correctly. This seems to work correctly locally, but I'm not sure I got a good repro. Pushing to verify the production test cases provided in T1076.

Auditors: vrana, btrahan

src/applications/differential/controller/changesetview/DifferentialChangesetViewController.php

@@ -230,6 +230,7 @@ final class DifferentialChangesetViewController extends DifferentialController {
     $detail = new DifferentialChangesetDetailView();
     $detail->setChangeset($changeset);
     $detail->appendChild($output);
+    $detail->setVsChangesetID($left_source);
 
     $detail->setRevisionID($request->getInt('revision_id'));