Make get_adjacent_post() wrap a new WP_Get_Adjacent_Post object that uses WP_Query.

See [27285] for the actual commit, which misfired.

props ethitter!
fixes #26937.

Built from https://develop.svn.wordpress.org/trunk@27286


git-svn-id: https://core.svn.wordpress.org/trunk@27142 1a063a9b-81f0-0310-95a4-ce76da25c4cd