Fix order in the references header when an in-reply-to msg-id is
erroneously added at the beginning instead of at the end. Add
description to the function that cleans up the reference headers for
threading.
Reported-by: Evan Gates <evan.gates@gmail.com>
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>