add "in replay to" headers to messages related to the same bug

after opening a new issue and sending the first email ("Created: xxx"), it will be nice that subsequent messages ("Commented:xxx", "Updated:xxx", "Closed:xxx", ...) could retain a reference to the first message, so that emails can be easily grouped in threads by email clients and mailing list managers (and google gmail).

This can be easily done adding the headers
In-Reply-To: (message id)
References: (previous messages list)
to messages