Explicitly track inline comment reply threading

Summary:
Ref T1460. Track and store which comments are threaded replies to other comments, vs merely appearing on the same lines.

This doesn't actually write `hasReplies` yet, since that needs to happen when we un-draft comments on submission.

Test Plan: Made inline comments in Differential and Diffusion, including replies. Replies were marked as "Is Reply".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1460

Differential Revision: https://secure.phabricator.com/D12017

src/applications/diffusion/controller/DiffusionInlineCommentController.php

@@ -43,6 +43,10 @@ final class DiffusionInlineCommentController
     return PhabricatorAuditInlineComment::loadID($id);
   }
 
+  protected function loadCommentByPHID($phid) {
+    return PhabricatorAuditInlineComment::loadPHID($phid);
+  }
+
   protected function loadCommentForEdit($id) {
     $request = $this->getRequest();
     $user = $request->getUser();