localrepository.status: only acquire wlock if actually needed. This speeds up the common case of not needing to update the dirstate, and avoids the need to reload and parse the dirstate "just in case".

To run the tests, do:

cd tests/
python run-tests.py

See http://www.selenic.com/mercurial/wiki/index.cgi/WritingTests for
more information on writing tests.