Using diffstat with the -m option gives the modified lines (as well as the added and deleted):

    diff dir1 dir2 | diffstat -m
[download]

I didn't need to modify it in any way. However, at the time of the original post I had to build the code from the website to get a version with the -m option. I think more recent OSes will come with that version by default or at least provide a package.

--
John.