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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.