http-backend: mark argc/argv unused

http-backend: mark argc/argv unused

We can't drop them because it's cmd_main(), which has a set prototype,
but the CGI interface does not do anything with such arguments.

Arguably we could detect them and complain. It's possible this could
detect misconfigurations or other mistakes, but:

  - as far as I can tell common webservers like apache do not have any
    mechanism to pass arguments to a CGI at all, so this isn't a mistake
    one could even make

  - it's possible that some obscure webserver might pass arguments, and
    we'd break that case. I have no idea if such a webserver exists; the
    CGI standard says only "The script is invoked in a system-defined
    manner".

So probably it would not hurt to detect them, but it also is unlikely to
help anyone. Let's just mark them as unused, which retains the current
behavior but silences -Wunused-parameter.

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