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/am.c

@@ -2180,6 +2180,8 @@ static int parse_opt_show_current_patch(const struct option *opt, const char *ar
 	};
 	int new_value = SHOW_PATCH_RAW;
 
+	BUG_ON_OPT_NEG(unset);
+
 	if (arg) {
 		for (new_value = 0; new_value < ARRAY_SIZE(valid_modes); new_value++) {
 			if (!strcmp(arg, valid_modes[new_value]))