code: def get_diff_trees(self, a_ref, b_ref=None):
          
          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
docs: 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)