Add sparse changes view

This is similar to https://smartgit.userecho.com/topics/159-changes-view-optionally-show-as-unified-diff/ as that unchanged lines are hidden. But is different from it because I still want two panes listing both files.

Sometimes there are small changes at the beginning and at the end of a big source file (add a method, add a import). Current changes view forces me to search for those tiny (they are really tiny for big files) side markers indicating a change. This is counterproductive if I want to get a quick impression on what has changed.