The ultimate `post_name` is stored in postmeta until the post is published. The `get_page_by_path()` function does not exclude `auto-draft` posts. Revert changes to `wp_unique_post_slug()` from [39411] which excluded `auto-draft` posts.
Props westonruter, dlh for testing, helen for testing.
Merges [39506] onto 4.7 branch.
Fixes#39078 for 4.7.
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