Add initial support for many promisor remotes

Add initial support for many promisor remotes

The promisor-remote.{c,h} files will contain functions to
manage many promisor remotes.

We expect that there will not be a lot of promisor remotes,
so it is ok to use a simple linked list to manage them.

Helped-by: Jeff King <peff@peff.net>
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>