CrClass->instance_select_query: return context_id of cr_item unless it is explicitly asked differently.
Before, calls like get_folder_children returned items having the context_id of the revision.
Since this attribute is ignored by most applications, it did not harm. However, a recent
bugfix for honoring different context_ids caused troubles. In the worst case, one can
end up with items having its own item_id as context id, which in turn causes
permission queries to loop.
