mingw: make mingw_signal return the correct handler

Returning the SIGALRM handler for SIGINT is not very useful.
Signed-off-by: NErik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -1677,14 +1677,16 @@ int sigaction(int sig, struct sigaction *in, struct sigaction *out)
 #undef signal
 sig_handler_t mingw_signal(int sig, sig_handler_t handler)
 {
-	sig_handler_t old = timer_fn;
+	sig_handler_t old;
 
 	switch (sig) {
 	case SIGALRM:
+		old = timer_fn;
 		timer_fn = handler;
 		break;
 
 	case SIGINT:
+		old = sigint_fn;
 		sigint_fn = handler;
 		break;