The create_append_path code didn't consider that list_concat will
modify it's first argument leading to inconsistent traversal of
resulting list.  In practice, it won't lead to any user-visible bug
but changing it for making the code behave consistently.

Reported-by: Tom Lane
Author: Tom Lane
Reviewed-by: Amit Khandekar and Amit Kapila
Discussion: https://postgr.es/m/32365.1528994120@sss.pgh.pa.us