For now diff is only made on products, workflow, portal_type,
category, portal_skin and action objects

Diff can be make with an installed business template if you just
select one business template, in this case it is done with the object
on the site. If you select two bt, diff is made between the original
objects in this bt


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@4325 20353a03-c40f-0410-a6d1-a30d3c3de9de