Patch Set 7:

I'll add tests for this in a separate change set.

I'm not quite sure if we should add this functionality to 'workspace model' because we compare there working tree against HEAD, not working tree against git index. Therefore putting some changes in index isn't straight forward behavior in this view.

IMHO enhancing compare editor from staging view should be also added in separate change, I could also work on this in  nearest feature ;)