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>

builtin-add.c

@@ -117,7 +117,7 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 			verbose = 1;
 			continue;
 		}
-		die(builtin_add_usage);
+		usage(builtin_add_usage);
 	}
 	pathspec = get_pathspec(prefix, argv + i);