rust-discovery: optimization of add commons/missings for empty arguments These two cases have to be catched early for different reasons. In the case of add_missing_revisions, we don't want to trigger the computation of the undecided set (and the children cache) too early: the later the better. In the case of add_common_revisions, the inner `MissingAncestors` object wouldn't know that all ancestors of its bases have already been removed from the undecided. In principle, that would in itself be a lead for further improvement: this remove_ancestors_from could be more incremental, but the current performance seems to be good enough. Differential Revision: https://phab.mercurial-scm.org/D6429