patch: make _addmodehdr a function under trydiff addmodehdr is a header helper, same as diffline, so it doesn't need to be a top-level function and can be nested under trydiff. In upcoming patches we will generalize this approach for all headers.

diff: rewrite diffline Make diffline more readable, using strings with placeholders rather than appending to a list from many ifs that makes difficult to understand the actual output format.