rust-discovery: takefullsample() core implementation take_full_sample() browses the undecided set in both directions: from its roots as well as from its heads. Following what's done on the Python side, we alter update_sample() signature to take a closure returning an iterator: either ParentsIterator or an iterator over the children found in `children_cache`. These constructs should probably be split off in a separate module. This is a first concrete example where a more abstract graph notion (probably a trait) would be useful, as this is nothing but an operation on the reversed DAG. A similar motivation in the context of the discovery process would be to replace the call to dagops::range in `add_missing_revisions()` with a simple iteration over descendents, again an operation on the reversed graph. Differential Revision: https://phab.mercurial-scm.org/D6424

rust-discovery: core implementation for take_quick_sample() This makes in particular `rand` no longer a testing dependency. We keep a seedable random generator on the `PartialDiscovery` object itself, to avoid lengthy initialization. In take_quick_sample() itself, we had to avoid keeping the reference to `self.undecided` to cope with the mutable reference introduced by the the call to `limit_sample`, but it's still manageable without resorting to inner mutability. Sampling being prone to be improved in the mid-term future, testing is minimal, amounting to checking which code path got executed. Differential Revision: https://phab.mercurial-scm.org/D6423

rust-discovery: read the index from a repo passed at init This makes the API of the Rust PartialDiscovery object now the same (or rather a subset) of the Python object, hence easier to control through module policy down the road. Differential Revision: https://phab.mercurial-scm.org/D6517