tests: test hg status --all with hgext/git
There's a TODO item in git/dirstate.py about obtaining clean files in a more
straightforward way, let's first test that status can and does show clean files
at all.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL