poll: honor the timeout on Win32

Ensure that when passing a pipe, the gnulib poll replacement will not
return 0 before the timeout has passed.

Not obeying the timeout (and merely returning 0) causes pathological
behavior when preparing a packfile for a repository and taking a
long time to do so.  If poll were to return 0 immediately, this would
cause keep-alives to get sent as quickly as possible until the packfile
was created.  Such deviance from the standard would cause megabytes (or
more) of keep-alive packets to be sent.

GetTickCount is used as it is efficient, stable and monotonically
increasing.  (Neither GetSystemTime nor QueryPerformanceCounter have
all three of these properties.)

compat/poll/poll.c

@@ -446,7 +446,7 @@ poll (struct pollfd *pfd, nfds_t nfd, int timeout)
   static HANDLE hEvent;
   WSANETWORKEVENTS ev;
   HANDLE h, handle_array[FD_SETSIZE + 2];
-  DWORD ret, wait_timeout, nhandles;
+  DWORD ret, wait_timeout, nhandles, start = 0, elapsed, orig_timeout = 0;
   fd_set rfds, wfds, xfds;
   BOOL poll_again;
   MSG msg;
@@ -459,6 +459,12 @@ poll (struct pollfd *pfd, nfds_t nfd, int timeout)
       return -1;
     }
 
+  if (timeout != INFTIM)
+    {
+      orig_timeout = timeout;
+      start = GetTickCount();
+    }
+
   if (!hEvent)
     hEvent = CreateEvent (NULL, FALSE, FALSE, NULL);
 
@@ -603,7 +609,13 @@ poll (struct pollfd *pfd, nfds_t nfd, int timeout)
 	rc++;
     }
 
-  if (!rc && timeout == INFTIM)
+  if (!rc && orig_timeout && timeout != INFTIM)
+    {
+      elapsed = GetTickCount() - start;
+      timeout = elapsed >= orig_timeout ? 0 : orig_timeout - elapsed;
+    }
+
+  if (!rc && timeout)
     {
       SleepEx (1, TRUE);
       goto restart;