Core Upgrader success statistics: Pass the version of WordPress we're upgrading from, as well as the version being upgraded to.

Merges [26016] and [26017] from 3.8 to the 3.7 branch.

fixes #25772.

Built from https://develop.svn.wordpress.org/branches/3.7@27883


git-svn-id: http://core.svn.wordpress.org/branches/3.7@27714 1a063a9b-81f0-0310-95a4-ce76da25c4cd