match: introduce nevermatcher for when no ignore files are present 952017471f93 introduced a deterministic `__repr__` for ignores. However, it didn't account for when ignore was `util.never`. This broke fsmonitor's ignore change detection -- with an empty hgignore, it would kick in all the time. Introduce `nevermatcher` and switch to it. This neatly parallels `alwaysmatcher`.