update_messages: Add old_stream_id variable for readability.
update_messages: Add old_stream_id variable for readability.

Having a parallel to new_stream_id seems better than accessing the
original event object directly for only one of these.

zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip