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.


git-svn-id: https://develop.svn.wordpress.org/branches/3.7@27883 602fd350-edb4-49c9-b593-d223f7449a82