revset: special case commonancestors(none()) to be empty set This matches the behavior of ancestor(none()). From an implementation perspective, ancestor() and commonancestors() are intersection, and ancestors() is union, so it would make some sense that commonancestors(none()) returned all revisions. However, ancestor(none()) isn't implemented as such, which breaks ancestor(x) == max(commonancestors(x)). From a user perspective, ancestors of nothing is nothing whichever type of operation the ancestor predicate does.