[MlMt] Identifying threads

[Ben Beuchler]

Here are headers from two messages that belong to the same thread but
MailMate is not threading:

Message A (comment added to an existing ticket but without replying to
a specific message in the ticket):

In-Reply-To:
References: <RT-Ticket-34476 at clockwork.net>
Message-ID: <rt-3.8.7-19240-1363188772-958.34476-8-0 at clockwork.net>


Message B (later in the same thread):

In-Reply-To: <rt-3.8.7-19240-1363188772-716.34476-5-0 at clockwork.net>
References: <RT-Ticket-34476 at clockwork.net>
<rt-3.8.7-19240-1363188772-716.34476-5-0 at clockwork.net>
Message-ID: <rt-3.8.7-20072-1363189351-1169.34476-5-0 at clockwork.net>

-Ben