http-backend: mark unused parameters in virtual functions

http-backend: mark unused parameters in virtual functions

The http-backend dispatches requests via a table of virtual functions.
Some of the functions ignore their "arg" parameter, because it's
implicit in the function (e.g., get_info_refs knows that it is
dispatched only for a request to "/info/refs").

Mark these unused parameters to silence -Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>