convert: clear the dirstate before a conversion, invalidate it afterwards
Clearing it before the conversion protects us from whatever data were
there (file copies in particular).
Invalidating it after the conversion avoids writing a possibly
inconsistent dirstate to disk.
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.