rust-index: add support for `_slicechunktodensity`

rust-index: headrevsfiltered() returning Rust result

rust-index: add support for `headrevsfiltered` The implementation is merged with that of `headrevs` also to make sure that caches are up to date.

rust-index: implement headrevs

rust-index: variant of assert_py_eq with normalizer expression The example given in doc-comment is the main use case: some methods may require ordering insensitive comparison. This is about to be used for `reachableroots2`

rust-index: add support for delta-chain computation

rust-index: add support for `find_snapshots`

rust-index: add `is_snapshot` method

rust-index: use the Rust index in `partialmatch`

rust-index: add missing special case for null rev This was an oversight, it was never a problem because we didn't use the index much for user-facing things in the past, which is the only real way of getting to this edge case.

rust-index: use the rust index in `shortest`

rust-index: add checks that `__contains__` is synchronized

rust-index: using the Rust index in nodemap updating methods

rust-index: implementation of __getitem__ Although the removed panic tends to prove if the full test suite did pass that the case when the input is a node id does not happen, it is best not to remove it right now. Raising IndexError is crucial for iteration on the index to stop, given the default CPython sequence iterator, see for instance https://github.com/zpoint/CPython-Internals/blobs/master/BasicObject/iter/iter.md This was spotted by `test-rust-ancestors.py`, which does simple interations on indexes (as preflight checks). In `revlog.c`, `index_getitem` defaults to `index_get` when called on revision numbers, which does raise `IndexError` with the same message as the one we are introducing here.

rust-index: optim note for post-scaffolding removal