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`.
Mercurial
=========
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