tests/test-hgrc

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).

#!/bin/sh

mkdir t
cd t
hg init
echo "invalid" > .hg/hgrc
hg status 2>&1 |sed -e "s:/.*\(/t/.*\):...\1:"