hook: detect & emit two more bugs

Trigger a bug when an unknown hook type is encountered while
setting up hook execution.

Also issue a bug if a configured hook is enabled without a cmd.

Mostly useful for defensive coding.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: 10ne1

hook.c

@@ -409,7 +409,11 @@ static int pick_next_hook(struct child_process *cp,
 	} else if (h->kind == HOOK_CONFIGURED) {
 		/* to enable oneliners, let config-specified hooks run in shell. */
 		cp->use_shell = true;
+		if (!h->u.configured.command)
+			BUG("non-disabled HOOK_CONFIGURED hook has no command");
 		strvec_push(&cp->args, h->u.configured.command);
+	} else {
+		BUG("unknown hook kind");
 	}
 
 	if (!cp->args.nr)