In retrospect, the loop prevention done by our indexer is not
always sufficient since it can have an improperly sorted
or incomplete References headers.
This bug was triggered multiple bracketed Message-IDs in an
In-Reply-To: header (not References) where the Message-IDs were
in non-chronological order when somebody tried to reply to
different leafs of a thread with a single message.
So we must check for descendents before blindly trying to
use the last one.
Fixes: c6a8fdf71e2c336f ("thread: last Reference always wins")