undo-files: cleanup legacy files when applicable
The "journal" code is much more compact in 6.4, and so is the "undo" files as a
result. However the previous version were much noisier, so let us cleanup undo
files from older version too.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.