interpret-trailers: mark unused "unset" parameters in option callbacks

interpret-trailers: mark unused "unset" parameters in option callbacks

There are a few parse-option callbacks that do not look at their "unset"
parameters, but also do not set PARSE_OPT_NONEG. At first glance this
seems like a bug, as we'd ignore "--no-if-exists", etc.

But they do work fine, because when "unset" is true, then "arg" is NULL.
And all three functions pass "arg" on to helper functions which do the
right thing with the NULL.

Note that this shortcut would not be correct if any callback used
PARSE_OPT_NOARG (in which case "arg" would be NULL but "unset" would be
false). But none of these do.

So the code is fine as-is. But we'll want to mark the unused "unset"
parameters to quiet -Wunused-parameter. I've also added a comment to
make this rather subtle situation more explicit.

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