parse-options: mark unused "opt" parameter in callbacks

The previous commit argued that parse-options callbacks should try to
use opt->value rather than touching globals directly. In some cases,
however, that's awkward to do. Some callbacks touch multiple variables,
or may even just call into an abstracted function that does so.

In some of these cases we _could_ convert them by stuffing the multiple
variables into a single struct and passing the struct pointer through
opt->value. But that may make other parts of the code less readable,
as the struct relationship has to be mentioned everywhere.

Let's just accept that these cases are special and leave them as-is. But
we do need to mark their "opt" parameters to satisfy -Wunused-parameter.

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

builtin/update-index.c

@@ -856,7 +856,7 @@ static int chmod_callback(const struct option *opt,
 	return 0;
 }
 
-static int resolve_undo_clear_callback(const struct option *opt,
+static int resolve_undo_clear_callback(const struct option *opt UNUSED,
 				const char *arg, int unset)
 {
 	BUG_ON_OPT_NEG(unset);
@@ -890,7 +890,7 @@ static int parse_new_style_cacheinfo(const char *arg,
 }
 
 static enum parse_opt_result cacheinfo_callback(
-	struct parse_opt_ctx_t *ctx, const struct option *opt,
+	struct parse_opt_ctx_t *ctx, const struct option *opt UNUSED,
 	const char *arg, int unset)
 {
 	struct object_id oid;