Revisions: Ensure the global `$post` remains the same after revision is restored.

Because `wp_insert_post()` and `wp_transition_post_status()` are called during the process, plugins can unexpectedly modify `$post`.

Props markparnell, tofandel, archon810, SergeyBiryukov.
Fixes #46671.
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git-svn-id: https://core.svn.wordpress.org/trunk@48387 1a063a9b-81f0-0310-95a4-ce76da25c4cd