gitk: showing onging merge as such ?

While resolving a conflicted merge, I wondered if there was any reason why
gitk does not show that we are indeed doing a merge, by showing the staged
contents as a merge commit if MERGE_HEAD is found.

Is there any reason why we would not do this ?

I had a quick try at how this could be done, here it is.  The diff is not
yet displayed as combined.