Handle invalid argc gently

describe, git: Handle argc==0 case the same way as argc==1.
merge-tree: Refuse excessive arguments.

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

merge-tree.c

@@ -337,7 +337,7 @@ int main(int argc, char **argv)
 	struct tree_desc t[3];
 	void *buf1, *buf2, *buf3;
 
-	if (argc < 4)
+	if (argc != 4)
 		usage(merge_tree_usage);
 
 	setup_git_directory();