Implement a basic --changed-group-format

etcupdate(8) requires that option, while GNU diff supports many more variation
of that options, their behaviour beside the simple verion implemented here are
quite inconsistent as such I do not plan to implement those.

The only special keyword supported by this implementation are: %< and %>
%= is not implemented as the documentation of GNU diff says: common lines, but
it actually when tested print the changes from the first file