mingw: correct exit-code for SIGALRM's SIG_DFL

Make sure SIG_DFL for SIGALRM exits with 128 + SIGALRM so other
processes can diagnose why it exits.

While we're at it, make sure we only write to stderr if it's a
terminal, and  change the output to match that of Linux.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1560,8 +1560,11 @@ static sig_handler_t timer_fn = SIG_DFL;
 static unsigned __stdcall ticktack(void *dummy)
 {
 	while (WaitForSingleObject(timer_event, timer_interval) == WAIT_TIMEOUT) {
-		if (timer_fn == SIG_DFL)
-			die("Alarm");
+		if (timer_fn == SIG_DFL) {
+			if (isatty(STDERR_FILENO))
+				fputs("Alarm clock\n", stderr);
+			exit(128 + SIGALRM);
+		}
 		if (timer_fn != SIG_IGN)
 			timer_fn(SIGALRM);
 		if (one_shot)