.mailmap: Map the the first submissions of MJG by e-mail

so that git shortlog with '-e' coalesces all my commits.
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.mailmap

@@ -36,6 +36,7 @@ Li Hong <leehong@pku.edu.cn>
 Lukas Sandström <lukass@etek.chalmers.se>
 Martin Langhoff <martin@catalyst.net.nz>
 Michael Coleman <tutufan@gmail.com>
+Michael J Gruber <git@drmicha.warpmail.net> <michaeljgruber+gmane@fastmail.fm>
 Michael W. Olson <mwolson@gnu.org>
 Michele Ballabio <barra_cuda@katamail.com>
 Nanako Shiraishi <nanako3@bluebottle.com>