In `wp_update_term()`, limit duplicate slug checks to the same taxonomy as the updated term.

In 4.1 [30240], `wp_insert_term()` was modified to allow the creation of terms
with duplicate slugs, as long as the terms are in different taxonomies.
`wp_update_term()` didn't get the corresponding modification, with the result
that term updates fail when the term being updated shares a slug with an older
term, regardless of that older term's taxonomy.

Props ipm-frommen.
Fixes #30780 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@30985 602fd350-edb4-49c9-b593-d223f7449a82