Fix add_partial_path interaction with disabled_nodes

Fix add_partial_path interaction with disabled_nodes

Commit e22253467942fdb100087787c3e1e3a8620c54b2 adjusted the logic in
add_path() to keep the path list sorted by disabled_nodes and then by
total_cost, but failed to make the corresponding adjustment to
add_partial_path. As a result, add_partial_path might sort the path list
just by total cost, which could lead to later planner misbehavior.

In principle, this should be back-patched to v18, but we are typically
reluctant to back-patch planner fixes for fear of destabilizing working
installations, and it is unclear to me that this has sufficiently
serious consequences to justify an exception, so for now, no back-patch.

Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Discussion: http://postgr.es/m/CAMbWs4-mO3jMK4t_LgcJ+7Eo=NmGgkxettgRaVbJzZvVZ1koMA@mail.gmail.com