Move the revisions upgrade handler to POST, to avoid esoteric metadata stomping.

props SergeyBiryukov.
see #25023.
for trunk.

Built from https://develop.svn.wordpress.org/trunk@25719


git-svn-id: http://core.svn.wordpress.org/trunk@25632 1a063a9b-81f0-0310-95a4-ce76da25c4cd