mailmap: consistently normalize brian m. carlson's name

v2.18.0-rc0~70^2 (mailmap: update brian m. carlson's email address,
2018-05-08) changed the mailmap to map

  sandals@crustytoothpaste.ath.cx
   -> brian m. carlson <sandals@crustytoothpaste.net>

instead of

  sandals@crustytoothpaste.net
    -> brian m. carlson <sandals@crustytoothpaste.ath.cx>

That means the mapping

  Brian M. Carlson <sandals@crustytoothpaste.ath.cx>
    -> brian m. carlson <sandals@crustytoothpaste.net>

is redundant, so we can remove it.  More importantly, it means that
the identity "Brian M. Carlson <sandals@crustytoothpaste.net>" used in
some commits is not normalized any more.  Add a mapping for it.

Noticed while updating Debian's Git packaging, which uses "git
shortlog --no-merges" to produce a list of changes in each version,
grouped by author's (normalized) name.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.mailmap

@@ -25,7 +25,7 @@ Ben Walton <bdwalton@gmail.com> <bwalton@artsci.utoronto.ca>
 Benoit Sigoure <tsunanet@gmail.com> <tsuna@lrde.epita.fr>
 Bernt Hansen <bernt@norang.ca> <bernt@alumni.uwaterloo.ca>
 Brandon Casey <drafnel@gmail.com> <casey@nrlssc.navy.mil>
-brian m. carlson <sandals@crustytoothpaste.net> Brian M. Carlson <sandals@crustytoothpaste.ath.cx>
+brian m. carlson <sandals@crustytoothpaste.net>
 brian m. carlson <sandals@crustytoothpaste.net> <sandals@crustytoothpaste.ath.cx>
 Bryan Larsen <bryan@larsen.st> <bryan.larsen@gmail.com>
 Bryan Larsen <bryan@larsen.st> <bryanlarsen@yahoo.com>