rebase: don't try to match -M option

The -M option does not exist in OPTIONS_SPEC, so there is no use to try
to find it.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -267,7 +267,7 @@ do
 	--no-fork-point)
 		fork_point=
 		;;
-	-M|-m)
+	-m)
 		do_merge=t
 		;;
 	-X)