Post revisions:

- Always update the revision version when updating post authors.
- Check if revisions have been updated and return early.
- Update the revisions by direct query to avoid resetting post_modified.
- Fix a bug where we may be comparing with an autosave but need to compare with the latest revision.

Fixes #16215.

git-svn-id: https://core.svn.wordpress.org/trunk@23849 1a063a9b-81f0-0310-95a4-ce76da25c4cd