Make report() from usage.c public as vreportf() and use it.

There exist already a number of static functions named 'report', therefore,
the function name was changed.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http-backend.c

@@ -538,14 +538,11 @@ static void service_rpc(char *service_name)
 
 static NORETURN void die_webcgi(const char *err, va_list params)
 {
-	char buffer[1000];
-
 	http_status(500, "Internal Server Error");
 	hdr_nocache();
 	end_headers();
 
-	vsnprintf(buffer, sizeof(buffer), err, params);
-	fprintf(stderr, "fatal: %s\n", buffer);
+	vreportf("fatal: ", err, params);
 	exit(0);
 }