Fix prologue of get_partition_ancestors()

Fix prologue of get_partition_ancestors()

The callers of this function assume that the first Oid in the list
returned by this function corresponds to the immediate parent and the
last on corresponds to the topmost parent.  Make that explicit in the
function prologue.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAExHW5vCbATEmht861=G-BFPHNwLUqyeGa_=8-xibJ6Q1UxAeA@mail.gmail.com