rust-index: use interior mutability in head revs and caches For upcoming changes in `hg-cpython` switching to the `hg-core` index in ancestors iterators, we will need to avoid excessive mutability, restricting the use of mutable references on `hg::index::Index` to methods that actually logically mutate it, whereas the maintenance of caches such as `head_revs` clearly does not. We illustrate that immediately by switching to immutable borrows in the corresponding methods of `hg-cpython::MixedIndex`