:

        - Preserve the wt.basis->wt.state changes
        - Transform the wt.basis to the new master basis.
        - Apply a merge of the old branch basis to get any 'local' changes from
          it into the tree.
        - Restore the wt.basis->wt.state changes.

        There isn't a single operation at the moment to do that, so we:

        - Merge current state -> basis tree of the master w.r.t. the old tree
          basis.
        - Do a 'normal' merge of the old branch basis if it is relevant.

        :param revision: The target revision to update to. Must be in the
            revision history.
        :param old_tip: If branch.update() has already been run, the value it
            returned (old tip of the branch or None). _marker is used
            otherwise.
        NF)r1