In commit 7b57e0e1 (Set Message-ID when sending package request emails,
2014-07-01), we changed the code responsible for sending notifications
such that the value of the Message-ID header is set deterministically in
the first email referring to a request. Unfortunately, this was
forgotten when porting the notification routines to Python in 9746a654
(Port notification routines to Python, 2015-06-27) and later fixed by
092e00f4 (notify: Fix references in request notifications, 2015-10-10).
However, when fixing another bug, the old behavior of not setting a
Message-ID was restored by d87b138a

 (notify: Fix merging of header dicts,
2015-10-26). Revert that particular change once more and add a comment
such that the line gets extra attention, should it be changed in the
future.

Fixes FS#48239.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>