`wp_insert_term()` does not allow for terms with the same name to exist
at the same hierarchy level, unless the second term has a unique slug.
When this logic was refactored in [31792] and [34809], a bug was
introduced whereby it was possible to bypass the no-same-named-sibling
check in cases where the first term had a non-auto-generated slug
(ie, where the name was 'Foo' but the slug something other than 'foo',
such that the second term would get the non-matching slug 'foo').
This changeset fixes this issue by ensuring that the duplicate name
check runs both in cases where there's an actual slug clash *and* in
cases where no explicit `slug` has been provided to `wp_insert_term()`.
The result is a more reliable error condition:
`wp_insert_term( 'Foo' ... )` will always fail if there's a sibling
'Foo', regardless of the sibling's slug.
Props mikejolley.
See #39984.
git-svn-id: https://develop.svn.wordpress.org/trunk@40144 602fd350-edb4-49c9-b593-d223f7449a82
8a2341335b