multi-pack-index: avoid writing to global in option callback

multi-pack-index: avoid writing to global in option callback

We declare the --object-dir option like:

  OPT_CALLBACK(0, "object-dir", &opts.object_dir, ...);

but the pointer to opts.object_dir is completely unused. Instead, the
callback writes directly to a global. Which fortunately happens to be
opts.object_dir. So everything works as expected, but it's unnecessarily
confusing.

Instead, let's have the callback write to the option value pointer that
has been passed in. This also quiets a -Wunused-parameter warning (since
we don't otherwise look at "opt").

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