tests/test-rebuildstate

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
# basic test for hg debugrebuildstate

hg init repo
cd repo

touch foo bar
hg ci -Am 'add foo bar'

touch baz
hg add baz
hg rm bar

hg debugrebuildstate
echo '% state dump after'
hg debugstate --nodates | sort
echo '% status'
hg st -A