dirstate-tree: Fix status algorithm with unreadable directory
When reading a directory fails such as because of insufficient permissions,
it should be treated as empty by status instead of skipped entirely.
Differential Revision: https://phab.mercurial-scm.org/D10823
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL