Add new hook_checkout_prepare, rollback earlier change of hook_checkout

In hindsight, it was a bad idea to allow manipulating the cart (entries)
in hook_checkout, because that hook is used by the `log` plugin. You now
get unused entries in the log.

Although that plugin should maybe have used hook_checkout_done, existing
log file readers (including scripts) and custom plugins may depend on
the CHECKOUT items in the log being before the BALANCE items.

lib/RevBank/Cart.pm

@@ -79,15 +79,18 @@ sub checkout($self, $user) {
 
     RevBank::FileIO::with_lock {
         my $transaction_id = time() - 1300000000;
+
+        RevBank::Plugins::call_hooks("checkout_prepare", $self, $user, $transaction_id);
+        for my $entry (@$entries) {
+            $entry->sanity_check;
+            $entry->user($user) if not $entry->user;
+        }
+
         RevBank::Plugins::call_hooks("checkout", $self, $user, $transaction_id);
 
         my %deltas = ($user => RevBank::Amount->new(0));
 
         for my $entry (@$entries) {
-            # In case entries were added by the hook
-            $entry->sanity_check;
-            $entry->user($user) if not $entry->user;
-
             $deltas{$_->{user}} += $_->{amount} * $entry->quantity
                 for $entry, $entry->contras;
         }

plugins/statiegeld_tokens

@@ -89,7 +89,7 @@ sub _handle_undo($cart) {
 	}
 }
 
-sub hook_checkout($class, $cart, $username, $transaction_id, @) {
+sub hook_checkout_prepare($class, $cart, $username, $transaction_id, @) {
 	if ($username eq '-undo') {
 		_handle_undo($cart);
 		return;