Transactions - adding willRenderTimeline to handle tricky cases

Summary: Fixes T6693.

Test Plan:
Made a bunch of comments on a diff with differential, being sure to leave inlines here and there. This reproduced the issue in T6693. With this patch this issue no longer reproduces!

Successfully "showed older changes" in Maniphest too.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6693

Differential Revision: https://secure.phabricator.com/D10931

src/applications/files/storage/PhabricatorFile.php

@@ -1095,6 +1095,13 @@ final class PhabricatorFile extends PhabricatorFileDAO
     return new PhabricatorFileTransaction();
   }
 
+  public function willRenderTimeline(
+    PhabricatorApplicationTransactionView $timeline,
+    AphrontRequest $request) {
+
+    return $timeline;
+  }
+
 
 /* -(  PhabricatorPolicyInterface Implementation  )-------------------------- */