The optimization in SVN rev 38556 seems to have changed
Popen.communicate's behavior when stdout is subprocess.PIPE (and maybe
for other cases as well).

See the attached file. In Python 2.4.5, all three counts are the same.
In Python 2.5.2, the middle count has increased by 1. In other words: A
file descriptor is leaked until the last reference to the Popen instance
is dropped.