parse-options.c: use exhaustive "case" arms for "enum parse_opt_result"

Change the "default" case in parse_options() that handles the return
value of parse_options_step() to simply have a "case" arm for
PARSE_OPT_UNKNOWN, instead of leaving it to a comment. This means the
compiler can warn us about any missing case arms.

This adjusts code added in ff43ec3e2d (parse-opt: create
parse_options_step., 2008-06-23), given its age it may pre-date the
existence (or widespread use) of this coding style, which we've since
adopted more widely.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.c

@@ -866,7 +866,7 @@ enum parse_opt_result parse_options(int argc, const char **argv,
 	case PARSE_OPT_NON_OPTION:
 	case PARSE_OPT_DONE:
 		break;
-	default: /* PARSE_OPT_UNKNOWN */
+	case PARSE_OPT_UNKNOWN:
 		if (ctx.argv[0][1] == '-') {
 			error(_("unknown option `%s'"), ctx.argv[0] + 2);
 		} else if (isascii(*ctx.opt)) {