Increase the pipe buffer size.

It commonly happens that git-fetch-pack and git-upload-pack hit a deadlock
in the initial commit id exchange, such that both try to write to the
other end, but do not succeed. I have the suspicion that the reason is
that both ends fill the pipe, but don't read.

Increasing the pipe buffer helps, but is this the real cure?

compat/mingw.c

@@ -106,7 +106,7 @@ int pipe(int filedes[2])
 	int fd;
 	HANDLE h[2], parent;
 
-	if (_pipe(filedes, 4096, 0) < 0)
+	if (_pipe(filedes, 8192, 0) < 0)
 		return -1;
 
 	parent = GetCurrentProcess();