This builds on the temporary backup system introduced in 6.3 to allow automatic updates to benefit from fatal error protection. A loopback request is performed to the home page of the site and the plugin is rolled back to its backed up version if a fatal error is observed.
For debugging and observability during beta, this change includes several calls to `error_log()` during the upgrade and rollback stages. These calls can be removed or placed behind a flag once we're ready for RC1.
Props costdev, johnbillion, mukesh27, afragen, audrasjb, justlevine, kirasong, peterwilsoncc
Fixes#58281
Built from https://develop.svn.wordpress.org/trunk@58128
git-svn-id: http://core.svn.wordpress.org/trunk@57593 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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