Update traversal_ids on parent change without save

Alex Pooleyrequested to merge
set_traversal_ids_on_parent_update into master

What does this MR do and why?

When trying to remove the use_traversal_ids_for_root_ancestor feature flag it was found that root_ancestor needed to update when a group parent was changed, but not saved.

This MR will return the correct root_ancestor when the group object's parent is changed and not saved.

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