promisor-remote: refactor accept_from_server()

promisor-remote: refactor accept_from_server()

In future commits, we are going to add more logic to
filter_promisor_remote() which is already doing a lot of things.

Let's alleviate that by moving the logic that checks and validates the
value of the `promisor.acceptFromServer` config variable into its own
accept_from_server() helper function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>