desired diff feature: show common lines

[Chris Konig Beatty]

Hi there

I would sometimes like see the common lines of two files only. I haven't 
been able to make diff show me that, might not be its purpose.

Would it be easily (enough) to add that functionality to diff?

I might miss the existence of some other tools. Obviously, there might 
not be enough demand for that feature anyway.

Thanks for your time and work!

Chris