Bugfix: 'stock' counted 1 after using 'repeat'

The 'stock' plugin uses the 'product_id' field in the cart item, but
this was not copied by the 'repeat' plugin. The confusion is caused by a
mismatch between the API for RevBank::Cart->add and the internal
representation.

lib/RevBank/Cart.pm

@@ -15,6 +15,10 @@ sub new {
 
 sub add {
     my ($self, $user, $amount, $description, $data) = @_;
+
+    # Note: 'repeat' plugin is currently dependent on this specific
+    # implementation!
+
     $data ||= {};
     my $item = {
         %$data,  # Internal stuff, not logged or printed.