rust: MissingAncestors.basesheads() This new API method on `MissingAncestors` leverages directly the Rust implementation for relative heads of a set, and also lowers the cost of returning the results to Python in the context of discovery. These interchange costs can probably be further reduced by implementing the `partialdiscovery` class in Rust, but that will be investigated in the 5.0 development cycle. Differential Revision: https://phab.mercurial-scm.org/D5584