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

Built from https://develop.svn.wordpress.org/trunk@25540


git-svn-id: http://core.svn.wordpress.org/trunk@25460 1a063a9b-81f0-0310-95a4-ce76da25c4cd