Content diff integration causing problems when other fields implement diff support

In file content.diff.inc there's a loop for all CCK fields in the nodes being compared and generates a row in $results even when field is not explicitly supported by this loop.

When a CCK field implements diff support, it generates its own results that are mixed by module_invoke_all used in diff module.

The attached patch fixes this. It generates results for diff module only for fields directly supported.