The primary use case is to warn unmatched paths. I originally thought paths in
negated expressions shouldn't be checked, but doing that seems rather
inconsistent than useful. For example, "~x" in "jj split '~x'" should match at
least one file to split to non-empty revisions.

Maybe we can optimize it to check paths during diffing, but I think it's okay
to add extra lookup cost at the end. The size of the path arguments is usually
small.

Closes jj-vcs#505