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


git-svn-id: https://develop.svn.wordpress.org/trunk@25540 602fd350-edb4-49c9-b593-d223f7449a82