replace usage of os.popen() with util.popen()
To make this possible, I added a mode parameter to both implementations of
util.popen(), defaulting to 'r' (as it does in the Python stdlib).
import doctest
import mercurial.changelog
# test doctest from changelog
doctest.testmod(mercurial.changelog)
import mercurial.httprepo
doctest.testmod(mercurial.httprepo)
import mercurial.util
doctest.testmod(mercurial.util)