mark "argv" as unused when we check argc

mark "argv" as unused when we check argc

A few commands don't take any options at all, and confirm this by
checking argc. After that they have no need to look at argv, but we're
still stuck with it by convention. Let's annotate these cases so that
the compiler doesn't complain with -Wunused-parameter.

Note that in scalar and get-tar-commit-id, we're forced to keep argv by
calling convention (the functions must match cmd_main() and builtin
cmd_foo() conventions, respectively). In diff, these are subcommand
modes that we call individually, so we _could_ just drop the argv
parameters entirely. But it's weird to pass argc without argv, and it
implies that the caller knows that the subcommands aren't interested in
further arguments. It's less confusing to just keep them and silence the
compiler warning.

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