docs: def delta_commits(self, baseline, target):
          """Show policies changes between arbitrary commits.
  
          The common use form is comparing the heads of two branches.
          """
          baseline_files = self._get_policy_fents(baseline.tree)
          target_files = self._get_policy_fents(target.tree)
  
          baseline_policies = PolicyCollection()
          target_policies = PolicyCollection()
  
          # Added
          for f in set(target_files) - set(baseline_files):
              target_policies += self._policy_file_rev(f, target)
  
          # Removed
          for f in set(baseline_files) - set(target_files):
              baseline_policies += self._policy_file_rev(f, baseline)
  
          # Modified
          for f in set(baseline_files).intersection(target_files):
              if baseline_files[f].hex == target_files[f].hex:
                  continue
              target_policies += self._policy_file_rev(f, target)
              baseline_policies += self._policy_file_rev(f, baseline)
  
          return CollectionDelta(
              baseline_policies, target_policies, target, self.repo_uri).delta()