rust-dirstate: improve API of `DirsMultiset`
- Use opaque `Iterator` type instead of implementation-specific one from
`HashMap`
- Make `DirsMultiset` behave like a set both in Rust and from Python
Differential Revision: https://phab.mercurial-scm.org/D6690
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t