If possible_ancestor is an ancestor of possible_descendant return the
child of possible_ancestor that is an ancestor of possible_descendant or
is possible_descendant itself.
When node_a and node_b share a least common ancestor, this function returns a
tuple containing the child of the least common ancestor that is an inclusive ancestor
of node_a and the child of the least common ancestor that is an inclusive ancestor
of node_b. If node_a and node_b do not have a least common ancestor, this
returns None.