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

  [email protected]
   -> brian m. carlson <[email protected]>

instead of

  [email protected]
    -> brian m. carlson <[email protected]>

That means the mapping

  Brian M. Carlson <[email protected]>
    -> brian m. carlson <[email protected]>

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

Author: jrn

.mailmap

@@ -25,7 +25,7 @@ Ben Walton <[email protected]> <[email protected]>
 Benoit Sigoure <[email protected]> <[email protected]>
 Bernt Hansen <[email protected]> <[email protected]>
 Brandon Casey <[email protected]> <[email protected]>
-brian m. carlson <[email protected]> Brian M. Carlson <[email protected]>
+brian m. carlson <[email protected]>
 brian m. carlson <[email protected]> <[email protected]>
 Bryan Larsen <[email protected]> <[email protected]>
 Bryan Larsen <[email protected]> <[email protected]>