diffs: move as_html and _safe_id from method to a pure function - avoid calling the method as function

as_html was sometimes used in a way where we actually don't want to use the
whole DiffProcessor - we just created a dummy instance and passed custom input
as parameter to the instance method.

Instead, make it the function we apparently want.

Make it clear that as_html not just returns a "diff" but that it is a html diff.