assert PARSE_OPT_NONEG in parse-options callbacks

In the spirit of 517fe807d6 (assert NOARG/NONEG behavior of
parse-options callbacks, 2018-11-05), let's cover some parse-options
callbacks which expect to be used with PARSE_OPT_NONEG but don't
explicitly assert that this is the case. These callbacks are all used
correctly in the current code, but this will help document their
expectations and future-proof the code.

As a bonus, it also silences -Wunused-parameters (these were added since
the initial sweep of 517fe807d6, and we can't yet turn on
-Wunused-parameters to remind people because it has too many existing
false positives).

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

builtin/commit-graph.c

@@ -128,6 +128,8 @@ static int write_option_parse_split(const struct option *opt, const char *arg,
 {
 	enum commit_graph_split_flags *flags = opt->value;
 
+	BUG_ON_OPT_NEG(unset);
+
 	opts.split = 1;
 	if (!arg)
 		return 0;