.mailmap: combine Stefan Beller's emails
Google mail has had the extension @googlemail.com for a long time
in Germany as @gmail.de was already taken by a competitor.
Nowadays the original gmail company isn't there anymore(?), hence
Googlemail also introduced @gmail.com in Germany, which I switched to.
This changed mail address of mine first appeared in
398dd4bd039680b
(2014-07-10, .mailmap: map different names with the same email
address together) ironically.
Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>