rust-index: support `unionrepo`'s compressed length hack Explanations inline.

rust-index: honour incoming using_general_delta in `deltachain` It looks to be a leftover from some past, but the C index considers only the value passed from Python whereas up to now the Rust index was using the value of its attribute. As a middle ground, we make this argument of `deltachain` optional from the Python side, with the Rust implementation only defaulting to its attribute. This way, we reduce false leads when a difference in results is spotted.

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`

rust-index: add Sync bound to all relevant mmap-derived values All readonly mmaps are Sync as far as Rust is concerned. Integrity of the mmap'ed file is a concern separate to Rust's memory model, since it requires out-of-program handling via locks, etc. This will help when we start sharing the Rust Index with Python.

debugindexstats: handle the lack of Rust support better We don't have any stats in the Rust index. Currently it is not known which stats would be interesting to get, so if they end up being important, we can add them later.

rust-python-index: don't panic on a corrupted index when calling from Python This makes `test-verify.t` pass again. In an ideal world, we would find the exact commit where this test breaks and amend part of this change there, but this is a long enough series.

tests: ignore test-storage when using Rust This is only relevant for Python code and the SQLite backend, which is in a half-abandoned state.

rust-index: optimize find_gca_candidates() on less than 8 revisions This is expected to be by far the most common case, given that, e.g., merging involves using it on two revisions. Using a `u8` as support for the bitset obviously divides the amount of RAM needed by 8. To state the obvious, on a repository with 10 million changesets, this spares 70MB. It is also possible that it'd be slightly faster, because it is easier to allocate and provides better cache locality. It is possible that some exhaustive listing of the traits implemented by `u8` and `u64` would avoid the added duplication, but that can be done later and would need a replacement for the `MAX` consts.

rust-index: simplification in find_gca_candidates() `parent_seen` can be made a mutable ref, making this part more obvious, not needing to be commented so much. The micro-optimization of avoiding the union if `parent_seen` and `current_seen` agree is pushed down in the `union()` method of the fast, `u64` based bit set implementation (in case it matters).

rust-index: avoid double negation in find_gca_candidates()

rust-index: avoid some cloning in find_gca_candidates() Instead of keeping the information whether the current revision is poisoned on `current_seen`, we extract it as a boolean. This also allows us to simplify the explanation of `seen[r].is_poisoned()`, as the exceptional case where it is poisoned right after `r` has been determined to be a solution does no longer exist.

rust-index: implement common_ancestors_heads() and ancestors() The only differences betwwen `common_ancestors_heads()` and `find_gca_candidates()` seems to be that: - the former accepts "overlapping" input revisions (meaning with duplicates). - limitation to 24 inputs (in the C code), that we translate to using the arbitrary size bit sets in the Rust code because we cannot bail to Python. Given that the input is expected to be small in most cases, we take the heavy handed approach of going through a HashSet and wait for perfomance assessment In case this is used via `hg-cpython`, we can anyway absorb the overhead by having `commonancestorheads` build a vector of unique values directly, and introduce a thin wrapper over `find_gca_candidates`, to take care of bit set type dispatching only. As far as `ancestors` is concerneed, this is just chaining `common_ancestors_heads()` with `find_deepest_revs`.

rust-index: find_gca_candidates bit sets genericization This allows to use arbitratry size of inputs in `find_gca_candidates()`. We're genericizing so that the common case of up to 63 inputs can be treated with the efficient implementation backed by `u64`. Some complications with the borrow checker came, because arbitrary sized bit sets will not be `Copy`, hence mutating them keeps a mut ref on the `seen` vector. This is solved by some cloning, most of which can be avoided, preferably in a follow-up after proof that this works (hence after exposition to Python layer). As far as performance is concerned, calling `clone()` on a `Copy` object (good case when number of revs is less than 64) should end up just doing a copy, according to this excerpt of the `Clone` trait documentation: Types that are Copy should have a trivial implementation of Clone. More formally: if T: Copy, x: T, and y: &T, then let x = y.clone(); is equivalent to let x = *y;. Manual implementations should be careful to uphold this invariant; however, unsafe code must not rely on it to ensure memory safety. We kept the general structure, hence why there are some double negations. This also could be made nicer in a follow-up. The `NonStaticPoisonableBitSet` is included to ensure that the `PoisonableBitSet` trait is general enough (had to correct `vec_of_empty()` for instance). Moving the genericization one level to encompass the `seen` vector and not its elements would be better for performance, if worth it.

rust-index: core impl for find_gca_candidates and find_deepest This still follows closely the C original and not able to treat more than 63 input revisions (bitset backed by `u64` and one bit reserved for poisoning).

rust-index: add support for `reachableroots2` Exposition in `hg-cpython` done in regular impl block, again for rustfmt support etc.

hg-cpython: rev_pyiter_collect_or_else It will be useful to give callers the control on the generated errors

rust-index: add support for `computephasesmapsets` Exposition in `hg-cpython` done in the regular `impl` block to enjoy rustfmt and clearer compilartion errors.

rust-index: slicechunktodensity returns Rust result Ready for removal of the scaffolding. This time, we allow ourselves a minor optimization: we avoid allocating for each chunk. Instead, we reuse the same vector, and perform at most one allocation per chunk. The `PyList` constructor will copy the buffer anyway.

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