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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def merge_tree(self, rhs, base=None):
 """Merge the given rhs treeish into the current index, possibly taking a common base treeish into account. As opposed to the from_tree_ method, this allows you to use an already existing tree as the left side of the merge :param rhs: treeish reference pointing to the 'other' side of the merge. :param base: optional treeish reference pointing to the common base of 'rhs' and this index which equals lhs :return: self ( containing the merge and possibly unmerged entries in case of conflicts ) :raise GitCommandError: If there is a merge conflict. The error will be raised at the first conflicting path. If you want to have proper merge resolution to be done by yourself, you have to commit the changed index ( or make a valid tree from it ) and retry with a three-way index.from_tree call. """

        # -i : ignore working tree status
        # --aggressive : handle more merge cases
        # -m : do an actual merge
        args = ["--aggressive", "-i", "-m"]
        if base is not None:
            args.append(base)
        args.append(rhs)

        self.repo.git.read_tree(args)
        return self