Rename the new 'left'/'right' flags on the `_wp_post_revision_field_$field` filter to 'from'/'to'.

Fixes #24848 for 3.6.

git-svn-id: https://core.svn.wordpress.org/branches/3.6@24836 1a063a9b-81f0-0310-95a4-ce76da25c4cd