Replace some calls to die(usage_str) with usage(usage_str).

The only change in behaviour should be having a "usage: " prefix
on the output string rather than "fatal: ", and an exit code of
129 rather than 128.

Signed-off-by: Ramsay Allan Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/builtin-add.c b/builtin-add.c
index f548b80..096b611 100644
--- a/builtin-add.c
+++ b/builtin-add.c
@@ -117,7 +117,7 @@
 			verbose = 1;
 			continue;
 		}
-		die(builtin_add_usage);
+		usage(builtin_add_usage);
 	}
 	pathspec = get_pathspec(prefix, argv + i);