Fix off-by-one error in git-merge

'git-merge -s' without a strategy name does not fail and does
not give usage as it should.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge.sh

@@ -43,7 +43,7 @@ do
 		case "$#,$1" in
 		*,*=*)
 			strategy=`expr "$1" : '-[^=]*=\(.*\)'` ;;
-		0,*)
+		1,*)
 			usage ;;
 		*)
 			strategy="$2"