usage.c API users: use die_message() for "fatal :" + exit 128

Change code that printed its own "fatal: " message and exited with a
status code of 128 to use the die_message() function added in a
preceding commit.

This change also demonstrates why the return value of
die_message_routine() needed to be that of "report_fn". We have
callers such as the run-command.c::child_err_spew() which would like
to replace its error routine with the return value of
"get_die_message_routine()".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -1075,6 +1075,6 @@ void NORETURN usage_msg_opt(const char *msg,
 		   const char * const *usagestr,
 		   const struct option *options)
 {
-	fprintf(stderr, "fatal: %s\n\n", msg);
+	die_message("%s\n", msg); /* The extra \n is intentional */
 	usage_with_options(usagestr, options);
 }