rust-revlog: add invalidation detection to `NodeTree` class This will be useful for callers, such as `scmutil` who reuse a `NodeTree` instance as a cache. They would otherwise get hard errors if any mutation of the index occurred since instantiation. This is something the C index does not provide.

rust-index: add support for `del index[r]` Only the `del index[r:]` syntax was supported, but the comment said otherwise. It's not actually used in core code, but the C index supports it.