Get the difference for this `channel_id` if any, then load entities.

        Calls :tl:`updates.getDifference`, which fills the entities cache
        (always done by `__call__`) and lets us know about the full entities.

async def _get_difference(self, channel_id, pts_date):
        """
        Get the difference for this `channel_id` if any, then load entities.

        Calls :tl:`updates.getDifference`, which fills the entities cache
        (always done by `__call__`) and lets us know about the full entities.
        """
        # Fetch since the last known pts/date before this update arrived,
        # in order to fetch this update at full, including its entities.
        self.client._log[__name__].debug('Getting difference for entities')
        if channel_id:
            try:
                where = await self.client.get_input_entity(channel_id)
            except ValueError:
                return

            result = await self.client(functions.updates.GetChannelDifferenceRequest(
                channel=where,
                filter=types.ChannelMessagesFilterEmpty(),
                pts=pts_date,  # just pts
                limit=100,
                force=True
            ))
        else:
            result = await self.client(functions.updates.GetDifferenceRequest(
                pts=pts_date[0],
                date=pts_date[1],
                qts=0
            ))

        if isinstance(result, (types.updates.Difference,
                               types.updates.DifferenceSlice,
                               types.updates.ChannelDifference,
                               types.updates.ChannelDifferenceTooLong)):
            self.original_update._entities.update({
                utils.get_peer_id(x): x for x in
                itertools.chain(result.users, result.chats)
            })

        if not self._load_entities():
            self.client._log[__name__].info(
                'Could not find all entities for update.pts = %s',
                getattr(self.original_update, 'pts', None)
            )