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-write-tree.c

@@ -73,7 +73,7 @@ int cmd_write_tree(int argc, const char **argv, const char *unused_prefix)
 		else if (!strncmp(arg, "--prefix=", 9))
 			prefix = arg + 9;
 		else
-			die(write_tree_usage);
+			usage(write_tree_usage);
 		argc--; argv++;
 	}