Note to self:
Grab a unified diff of the new version versus the old version:
svn diff -r HEAD --old http://svn.automattic.com/wordpress/tags/<old_version> \
--new http://svn.automattic.com/wordpress/tags/<new_version>
Apply the changes to the current installation:
patch -d <path_to_wp_install> -u -i <patchfile>