undo-files: relies on a explicit list of possible undo files Instead of infering the list of undo files from the `_journalfiles` method on `localrepository`, we explicitly have the list of file in a constant next to the cleanup code. In practice this does not change much as `_journalfiles` is already returning the same "static" list and no internal or extensions extensions seems to actually wrap that. In addition, that list is not "too short" for cleanup, in case we need to cleanup undo files from older version of Mercurial that used to use more of them. this will be dealt with in a later changesets. This change is a step toward our goal to use the `cleanup_undo_files` within the transaction. The transaction has no reference to the `repo` object, so we need to move toward `cleanup_undo_files` not having one either.