match: pass 'warn' argument to _normalize() for consistency
No other arguments are passed via the matcher's state, so we should
treat 'warn' the consistently. More importantly, this will let us make
it a static function, which will help with further refactoring.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html