Upgrader: Perform a MD5 file verification check on the files during upgrade. This ensures that both a Partial upgrade build can be used, and that all the files were copied into place correctly.
Props pento for initial patch. Fixes #18201

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When using download_url(), if the resource supplies a Content-MD5 header, verify the downloaded file against it. Fixes #20074

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WordPress Core Automatic Updates: Add a filter as to whether or not to send the site administrator a summary of executed updates. Props JustinSainton. See #22704

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