git-merge: show usage if run without arguments

Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-merge.sh

@@ -93,6 +93,8 @@ finish () {
 	esac
 }
 
+case "$#" in 0) usage ;; esac
+
 rloga=
 while case "$#" in 0) break ;; esac
 do