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
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