rust-matchers: add `FileMatcher` implementation
Mercurial defines an `exactmatcher`, I find `FileMatcher` to be clearer, but
am not opposed to using the old name.
This change also switched the order of `assert_eq` arguments as it is clearer
that way for most people.
Differential Revision: https://phab.mercurial-scm.org/D7528
mercurial (__VERSION__) __CODENAME__; urgency=medium
* Automated build performed by upstream.
-- Mercurial Devel <mercurial-devel@mercurial-scm.org> __DATE__