Windows: Implement asynchronous functions as threads.

In upload-pack we must explicitly close the output channel of rev-list.
(On Unix, the channel is closed automatically because process that runs
rev-list terminates.)

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

run-command.h

@@ -76,7 +76,12 @@ struct async {
 	int (*proc)(int fd, void *data);
 	void *data;
 	int out;	/* caller reads from here and closes it */
+#ifndef __MINGW32__
 	pid_t pid;
+#else
+	HANDLE tid;
+	int fd_for_proc;
+#endif
 };
 
 int start_async(struct async *async);