Fix stack overflow from faulty headers that cause a circularity in the
threading algorithm. Remove duplicated message-ids in the references
headers. Check that the message-id itself is not part of the references.
Fixes: https://todo.sr.ht/~rjarry/aerc/32
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Robin Jarry <robin@jarry.cc>
Acked-by: Robin Jarry <robin@jarry.cc>