Add `current-cat-ancestor` class to ancestor items in `wp_list_categories()`.

Pairs nicely with `current-cat-parent`.

Props jrchamp, swisssipdy, ardathksheyna, wonderboymusic.
Fixes #10676.
Built from https://develop.svn.wordpress.org/trunk@36008


git-svn-id: https://core.svn.wordpress.org/trunk@35973 1a063a9b-81f0-0310-95a4-ce76da25c4cd