mailmap: add an additional email address for brian m. carlson

To more accurately track the provenance of contributions, brian uses a
work email address for commits created at work. Add this email address
to .mailmap so that contributions are properly attributed to the same
person.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.mailmap

@@ -31,6 +31,7 @@ Brandon Casey <drafnel@gmail.com> <casey@nrlssc.navy.mil>
 Brandon Williams <bwilliams.eng@gmail.com> <bmwill@google.com>
 brian m. carlson <sandals@crustytoothpaste.net>
 brian m. carlson <sandals@crustytoothpaste.net> <sandals@crustytoothpaste.ath.cx>
+brian m. carlson <sandals@crustytoothpaste.net> <bk2204@github.com>
 Bryan Larsen <bryan@larsen.st> <bryan.larsen@gmail.com>
 Bryan Larsen <bryan@larsen.st> <bryanlarsen@yahoo.com>
 Cheng Renquan <crquan@gmail.com>