mingw: support waitpid with pid > 0 and WNOHANG

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

compat/mingw.c

@@ -1554,6 +1554,14 @@ pid_t waitpid(pid_t pid, int *status, unsigned options)
 		return -1;
 	}
 
+	if (pid > 0 && options & WNOHANG) {
+		if (WAIT_OBJECT_0 != WaitForSingleObject(h, 0)) {
+			CloseHandle(h);
+			return 0;
+		}
+		options &= ~WNOHANG;
+	}
+
 	if (options == 0) {
 		struct pinfo_t **ppinfo;
 		if (WaitForSingleObject(h, INFINITE) != WAIT_OBJECT_0) {

compat/mingw.h

@@ -140,6 +140,7 @@ static inline int mingw_unlink(const char *pathname)
 }
 #define unlink mingw_unlink
 
+#define WNOHANG 1
 pid_t waitpid(pid_t pid, int *status, unsigned options);
 
 #ifndef NO_OPENSSL