[1133112] Inventory Merge is slow and consumes a lot of memory when a parent has many children
In our continuing search for new planets, new civilizations, er, strike that,
our need for better performance, this change ensures we don't pull
child sets unnecessarily during inventory merge.  This is very important in
the given use case, but should benefit all inventory merge to some degree.

(cherry picked from commit bdeeef8ab36aed0d7d1f38dcb9d19c8cc8150ca0)
Signed-off-by: Thomas Segismont <tsegismo@redhat.com>