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>