Add an action just prior to updating an edited post

[tellyworth]

This patch adds a new 'pre_post_update' action that runs just prior to saving an edited post.

Sample scenario: a plugin needs to notify an external web service of changes to posts. By the time edit_post and transition status actions run, the contents of the post prior to saving are no longer known - including the old URL, which may have changed if post_status or post_name were edited. The new action gives plugins a chance to fetch any post fields they need before they're changed.

If there's a better way to do it I'm happy to hear of it.

(In [6938]) pre_post_update action from tellyworth. fixes #5815