Call wp_update_term_count() only for those terms that have been added to or...

Call wp_update_term_count() only for those terms that have been added to or removed from the object.  Props jeremyclarke. fixes #18631

git-svn-id: https://core.svn.wordpress.org/trunk@18999 1a063a9b-81f0-0310-95a4-ce76da25c4cd