Call _get_post_ancestors() from get_post_ancestors() if the ancestors property...

Call _get_post_ancestors() from get_post_ancestors() if the ancestors property is not set in the post object. Works around situations where ancestors is not set in the cached version of the post object. see #18536

git-svn-id: https://core.svn.wordpress.org/trunk@20171 1a063a9b-81f0-0310-95a4-ce76da25c4cd