When replying to a ghost comment, attach the reply to the same place

Summary:
Fixes T10562. I left this behavior sort of ambiguous in the original implementation because I didn't anticipate or stumble across this situation.

It's easy to fix: when you reply to a ghost, just put the reply in the exact same place as the ghost (even if it's a different diff), so they always move/ghost/port/thread together.

Test Plan:
See T10562 for reproduction steps and a "before" picture. Here's the after picture:

{F1168983}

The two comments at the bottom are pre-fix, and exhibit the bug. The comment at the top is post-fix, and appears adjacent to the original correctly.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T10562

Differential Revision: https://secure.phabricator.com/D15458

src/infrastructure/diff/PhabricatorInlineCommentController.php

@@ -305,13 +305,18 @@ abstract class PhabricatorInlineCommentController
           pht('Failed to load comment "%s".', $reply_phid));
       }
 
-      // NOTE: It's fine to reply to a comment from a different changeset, so
-      // the reply comment may not appear on the same changeset that the new
-      // comment appears on. This is expected in the case of ghost comments.
-      // We currently put the new comment on the visible changeset, not the
-      // original comment's changeset.
+      // When replying, force the new comment into the same location as the
+      // old comment. If we don't do this, replying to a ghost comment from
+      // diff A while viewing diff B can end up placing the two comments in
+      // different places while viewing diff C, because the porting algorithm
+      // makes a different decision. Forcing the comments to bind to the same
+      // place makes sure they stick together no matter which diff is being
+      // viewed. See T10562 for discussion.
 
+      $this->changesetID = $reply_comment->getChangesetID();
       $this->isNewFile = $reply_comment->getIsNewFile();
+      $this->lineNumber = $reply_comment->getLineNumber();
+      $this->lineLength = $reply_comment->getLineLength();
     }
   }