serve: mark unused parameters in virtual functions

serve: mark unused parameters in virtual functions

Each v2 "serve" action has a virtual function for advertising and
implementing the command. A few of these are so trivial that they don't
need to look at their parameters, especially the "repository" parameter.
We can mark them so that -Wunused-parameter doesn't complain.

Note that upload_pack_v2() probably _should_ be using its repository
pointer. But teaching the functions it calls to do so is non-trivial.
Even using it for something as simple as reading config is tricky, both
because it shares code with the v1 upload pack, and because the
git_protected_config() mechanism it uses does not have a repo-specific
interface. So we'll just annotate it for now, and cleaning it up can be
part of the larger work to drop references to the_repository.

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