remote-curl: reduce scope of rpc_state.stdin_preamble

remote-curl: reduce scope of rpc_state.stdin_preamble

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: Jonathan Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>