Method for getting two repo trees between two git tag commits
        returns the dvc hash names of changed file/directory

        Args:
            a_ref(str) - git reference
            b_ref(str) - optional second git reference, default None

        Returns:
            dict - dictionary with keys: (a_tree, b_tree, a_ref, b_ref, equal)

def get_diff_trees(self, a_ref, b_ref=None):
        """Method for getting two repo trees between two git tag commits
        returns the dvc hash names of changed file/directory

        Args:
            a_ref(str) - git reference
            b_ref(str) - optional second git reference, default None

        Returns:
            dict - dictionary with keys: (a_tree, b_tree, a_ref, b_ref, equal)
        """
        diff_dct = {DIFF_EQUAL: False}
        trees, commit_refs = self._get_diff_trees(a_ref, b_ref)
        diff_dct[DIFF_A_REF] = commit_refs[0]
        diff_dct[DIFF_B_REF] = commit_refs[1]
        if commit_refs[0] == commit_refs[1]:
            diff_dct[DIFF_EQUAL] = True
            return diff_dct
        diff_dct[DIFF_A_TREE] = trees[DIFF_A_TREE]
        diff_dct[DIFF_B_TREE] = trees[DIFF_B_TREE]
        return diff_dct