commit: avoid writing to global in option callback

commit: avoid writing to global in option callback

The callback function for --trailer writes directly to the global
trailer_args and ignores opt->value completely. This is OK, since that's
where we expect to find the value. But it does mean the option
declaration isn't as clear. E.g., we have:

    OPT_BOOL(0, "reset-author", &renew_authorship, ...),
    OPT_CALLBACK_F(0, "trailer", NULL, ..., opt_pass_trailer)

In the first one we can see where the result will be stored, but in the
second, we get only NULL, and you have to go read the callback.

Let's pass &trailer_args, and use it in the callback. As a bonus, this
silences a -Wunused-parameter warning.

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