rust-cpython: bindings for MissingAncestors The exposition is rather straightforward, except for the remove_ancestors_from() method, which forces us to an inefficient conversion between Python sets and Rust HashSets. Two alternatives are proposed in comments: - changing the inner API to "emit" the revision numbers to discard this would be a substantial change, and it would be better only in the cases where there are more to retain than to discard - mutating the Python set directly: this would force us to define an abstract `RevisionSet` trait, and implement it both for plain `HashSet` and for a struct enclosing a Python set with the GIL marker `Python<'p>`, also a non trivial effort. The main (and seemingly only) caller of this method being `mercurial.setdiscovery`, which is currently undergoing serious refactoring, it's not clear whether these improvements would be worth the effort right now, so we're leaving it as-is. Also, in `get_bases()` (will also be used by `setdiscovery`), we'd prefer to build a Python set directly, but we resort to returning a tuple, waiting to hear back from our PR onto rust-cpython about that Differential Revision: https://phab.mercurial-scm.org/D5550