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.

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

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.