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.

revset: clarify heads() order doesn't matter while computing common ancestors Follows up 5460926352ee and 52f19a840543.

hghave: require clang-format >= 6 due to output change