bisect--helper: fix cmd_*() function switch default return

In a `cmd_*()` function, return `error()` cannot be used
because that translates to `-1` and `cmd_*()` functions need
to return exit codes.

Let's fix switch default return.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/bisect--helper.c

@@ -711,7 +711,7 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 		res = bisect_start(&terms, no_checkout, argv, argc);
 		break;
 	default:
-		return error("BUG: unknown subcommand '%d'", cmdmode);
+		BUG("unknown subcommand %d", (int)cmdmode);
 	}
 	free_terms(&terms);