remote: move static variables into per-repository struct

remote: move static variables into per-repository struct

remote.c does not works with non-the_repository because it stores its
state as static variables. To support non-the_repository, we can use a
per-repository struct for the remotes subsystem.

Prepare for this change by defining a struct remote_state that holds
the remotes subsystem state and move the static variables of remote.c
into the_repository->remote_state.

This introduces no behavioral or API changes.

Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>