Further clean-up: usage() vs die()

This hopefully finishes the clean-up Ramsay started with recent
commit 15e593e4d37d1d350fef20ab666d58f6881c7f5f and commit
8cdf33643dc0b21d9ea922a3fdd7f64226c421aa.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/builtin-mv.c b/builtin-mv.c
index 62ae937..e47942c 100644
--- a/builtin-mv.c
+++ b/builtin-mv.c
@@ -99,7 +99,7 @@
 			ignore_errors = 1;
 			continue;
 		}
-		die(builtin_mv_usage);
+		usage(builtin_mv_usage);
 	}
 	count = argc - i - 1;
 	if (count < 1)