thread: prevent hidden threads in /$INBOX/ landing page
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")