diff --no-index: make the usage string less scary

Start the diff --no-index usage string with "usage:" instead of
"fatal:".
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -201,8 +201,8 @@ void diff_no_index(struct rev_info *revs,
 			return;
 	}
 	if (argc != i + 2)
-		die("git diff %s takes two paths",
-		    no_index ? "--no-index" : "[--no-index]");
+		usagef("git diff %s <path> <path>",
+		       no_index ? "--no-index" : "[--no-index]");
 
 	diff_setup(&revs->diffopt);
 	for (i = 1; i < argc - 2; ) {