After prune_if_smaller_than is run, we may still have excess
        nodes.

        For example, with a small_size of 609710690:

                     7  /*
              28815419  /data/*
                    32  /data/srv/*
                925746  /data/srv/docker.bak/*
                    12  /data/srv/docker.bak/shared/*
             682860348  /data/srv/docker.bak/shared/standalone/*

        This is reduced to:

              31147487  /*
             682860355  /data/srv/docker.bak/shared/standalone/*

        Run this only when done with the scanning.

def merge_upwards_if_smaller_than(self, small_size, a_or_u):
        """After prune_if_smaller_than is run, we may still have excess
        nodes.

        For example, with a small_size of 609710690:

                     7  /*
              28815419  /data/*
                    32  /data/srv/*
                925746  /data/srv/docker.bak/*
                    12  /data/srv/docker.bak/shared/*
             682860348  /data/srv/docker.bak/shared/standalone/*

        This is reduced to:

              31147487  /*
             682860355  /data/srv/docker.bak/shared/standalone/*

        Run this only when done with the scanning."""

        # Assert that we're not messing things up.
        prev_app_size = self.app_size()
        prev_use_size = self.use_size()

        small_nodes = self._find_small_nodes(small_size, (), a_or_u)
        for node, parents in small_nodes:
            # Check immediate grandparent for isdir=None and if it
            # exists, move this there. The isdir=None node is always
            # last.
            if len(parents) >= 2:
                tail = parents[-2]._nodes[-1]
                if tail._isdir is None:
                    assert tail._app_size is not None, tail
                    tail._add_size(node.app_size(), node.use_size())
                    parents[-1]._nodes.remove(node)
                    assert len(parents[-1]._nodes)

        # The actual assertion.
        assert prev_app_size == self.app_size(), (
            prev_app_size, self.app_size())
        assert prev_use_size == self.use_size(), (
            prev_use_size, self.use_size())