In `get_page_children()`, only check `$page->ancestors` once to avoid duplicates when the function recurses. Adds an argument, `$ancestors`.

Fixes #18962.

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Twenty Fifteen: Our next-previous labels were reversed.

Props iamtakashi, fixes #30265.


Built from https://develop.svn.wordpress.org/trunk@30247


git-svn-id: http://core.svn.wordpress.org/trunk@30247 1a063a9b-81f0-0310-95a4-ce76da25c4cd