rust-dirs-multiset: add `DirsChildrenMultiset`
In a future patch, this structure will be needed to store information needed by
the (also upcoming) `IgnoreMatcher`.
Differential Revision: https://phab.mercurial-scm.org/D7869
rust-hg-path: add useful methods to `HgPath`
This changeset introduces the use of the `pretty_assertions` crate for easier
to read test output.
Differential Revision: https://phab.mercurial-scm.org/D7867