Preserve mutation changes when updating parent_id

Author: schovi

lib/closure_tree/hierarchy_maintenance.rb

@@ -42,10 +42,20 @@ def _ct_after_save
         rebuild!
       end
       if public_send(changes_method)[_ct.parent_column_name] && !@was_new_record
+        tmp_previous_mutation_tracker = @previous_mutation_tracker
+        tmp_mutation_tracker = @mutation_tracker
+        tmp_mutations_from_database = @mutations_from_database
+        tmp_mutations_before_last_save = @mutations_before_last_save
+
         # Resetting the ancestral collections addresses
         # https://github.com/mceachen/closure_tree/issues/68
         ancestor_hierarchies.reload
         self_and_ancestors.reload
+
+        @previous_mutation_tracker = tmp_previous_mutation_tracker
+        @mutation_tracker = tmp_mutation_tracker
+        @mutations_from_database = tmp_mutations_from_database
+        @mutations_before_last_save = tmp_mutations_before_last_save
       end
       @was_new_record = false # we aren't new anymore.
       @_ct_skip_sort_order_maintenance = false # only skip once.