Add shadow group members to the group if gshadow is used.

    Normally group and shadow should be in sync, but no guarantees. Merges the
    two stores as membership in either file may confer membership.

def MergeMembers(self):
    """Add shadow group members to the group if gshadow is used.

    Normally group and shadow should be in sync, but no guarantees. Merges the
    two stores as membership in either file may confer membership.
    """
    for group_name, members in iteritems(self.gshadow_members):
      group = self.entry.get(group_name)
      if group and group.pw_entry.store == self.shadow_store:
        group.members = members.union(group.members)