mailmap: consistently normalize brian m. carlson's name

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 <redacted>

instead of

  sandals@crustytoothpaste.net
    -> brian m. carlson <redacted>

That means the mapping

  Brian M. Carlson <redacted>
    -> brian m. carlson <redacted>

is redundant, so we can remove it.  More importantly, it means that
the identity "Brian M. Carlson <redacted>" 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 <redacted>
Signed-off-by: Junio C Hamano <redacted>