promisor-remote: change promisor_remote_reply()'s signature

promisor-remote: change promisor_remote_reply()'s signature

The `promisor_remote_reply()` function performs two tasks:
1. It uses filter_promisor_remote() to parse the server's
   "promisor-remote" advertisement and to mark accepted remotes in the
   repository configuration.
2. It assembles a reply string containing the accepted remote names to
   send back to the server.

In a following commit, the fetch-pack logic will need to trigger the
side effect (1) to ensure the repository state is correct, but it will
not need to send a reply (2).

To avoid assembling a reply string when it is not needed, let's change
the signature of promisor_remote_reply(). It will now return `void` and
accept a second `char **accepted_out` argument. Only if that argument
is not NULL will a reply string be assembled and returned back to the
caller via that argument.

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