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

rust-index: check that the entry bytes are the same in both indexes This is a temporary measure to show that both the Rust and C indexes are kept in sync. Comes with some related documentation precisions. For comparison of error cases, see `index_entry_binary()` in `revlog.c`.

rust-index: return variables systematic naming convention To help knowing at a glance when a method is ready, making us more comofortable when we are close to the final removal of scaffolding, we introduce the systematic variable names `rust_res` and `c_res`. The goal of this series is to always return the formet. We take again the case of `pack_header` as example. Our personal opinion is to usually avoid such poor semantics as `res`, but usually accept it when it close to the actual return, which will be the case in most methods of this series. Also, the name can simply be dropped when we remove the scaffolding. To follow on the example, the body of `pack_header()` should become this in the final version: ``` let index = self.index(py).borrow(); let packed = index.pack_header(args.get_item(py, 0).extract(py)?); Ok(PyBytes::new(py, &packed).into_object()); ``` in these cases it is close to the actual return and will be removed at the end entirely.