rc diff: evaluate diff-highlight hook before loading module
rc diff: evaluate diff-highlight hook before loading module

A recent commit wrapped diff.kak into a module. The module includes the
hook that adds diff highlighting to filetype=diff buffers.  This means
that the hook is only loaded after opening the first diff buffer in a
Kakoune session, so it only actually fires for the second diff buffer.
Fix this by moving the hook out of the module.

Fixes #4525

mawww/kakoune,mawww/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,jkonecny12/kakoune,casimir/kakoune,casimir/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,casimir/kakoune