Diffusion - be sure to properly unserialize result from conduit query

Summary: Fixes T7256.

Test Plan: Looked at rXPRF0a7a5f69f5d7 in a local instance. things looked great both pre and post patch.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7256

Differential Revision: https://secure.phabricator.com/D11790

src/applications/diffusion/controller/DiffusionCommitController.php

@@ -894,6 +894,7 @@ final class DiffusionCommitController extends DiffusionController {
     if (!$merges) {
       return null;
     }
+    $merges = DiffusionPathChange::newFromConduit($merges);
 
     $caption = null;
     if (count($merges) > $limit) {