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.
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