mailmap: use Michael J Gruber's new address

Map both old addresses to the new, hopefully more permanent one.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: mjg

.mailmap

@@ -151,7 +151,8 @@ Matthias Kestenholz <matthias@spinlock.ch> <mk@spinlock.ch>
 Matthias Urlichs <matthias@urlichs.de> <smurf@kiste.(none)>
 Matthias Urlichs <matthias@urlichs.de> <smurf@smurf.noris.de>
 Michael Coleman <tutufan@gmail.com>
-Michael J Gruber <git@drmicha.warpmail.net> <michaeljgruber+gmane@fastmail.fm>
+Michael J Gruber <git@grubix.eu> <michaeljgruber+gmane@fastmail.fm>
+Michael J Gruber <git@grubix.eu> <git@drmicha.warpmail.net>
 Michael S. Tsirkin <mst@kernel.org> <mst@redhat.com>
 Michael S. Tsirkin <mst@kernel.org> <mst@mellanox.co.il>
 Michael S. Tsirkin <mst@kernel.org> <mst@dev.mellanox.co.il>