bisect--helper: BUG() in cmd_*() on invalid subcommand

In cmd_bisect__helper() function, if an invalid or no
subcommand is passed there is a BUG.

BUG() out instead of returning an error.

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

@@ -720,7 +720,7 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 		res = bisect_start(&terms, argv, argc);
 		break;
 	default:
-		return error("BUG: unknown subcommand '%d'", cmdmode);
+		BUG("unknown subcommand %d", cmdmode);
 	}
 	free_terms(&terms);