The stdin_preamble field in struct rpc_state is only used in
rpc_service(), and not in any functions it directly or indirectly calls.
Refactor it to become an argument of rpc_service() instead.

An observation of all callers of rpc_service() shows that the preamble
is always set, so we no longer need the "if (preamble)" check.
Signed-off-by: NJonathan Tan <jonathantanmy@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>