.mailmap: map different names with the same email address together

Pretty much one year ago (94b410bba864, Jul 12 2013, .mailmap: Map
email addresses to names) I cleaned up the output of `git shortlog
-sne` of git.git by writing a .mailmap file fot the git.git project.

During the year Jens, Kazuki and Trần contributed to git.git using
different names, but the same email address; unify them.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>