.arcconfig

rust-matchers: improve `Matcher` trait ergonomics

`VisitChildrenSet` has no need to own the set, this will save allocations.

The `file_set` return type change is motivated by both ergonomics and... being
able to compile code.
The `AlwaysMatcher` does not store a `file_set`, which requires it to return an
owned `HashSet`, which in turn would change our return type to `Cow<&HgPath>`
(lifetimes omitted). This is both un-ergonomic and troublesome for more
complex lifetime issues (especially with the upcoming `FileMatcher` in the
following patch).

Differential Revision: https://phab.mercurial-scm.org/D7525

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