Show patch in gitk --first-parent ?

Hi all,

With "gitk --first-parent", I get a graph that only follows the first parent of every merge---perfect if you always merge topic branches into the main branch. However, the diff shown is still the normal merge diff, and not the diff between the new tree and the first parent's tree---is it possible to make it do that instead?

(With "git log --first-parent -p -m", I get precisely the diffs I want, but everything is fully expanded so it's hard to navigate.)