Not possible to see combined diff before committing a merge?

Am I missing something, or is there no way to see combined diff output after completing merge resolution on a conflicted merge but before committing the merge?

$ git diff --cc [-- path]

works fine _until_ you've added the file to the index, at which point you're hosed.

I think I want the combined diff between:

- What's in the index
- The first parent (ORIG_HEAD)
- The second parent (MERGE_HEAD)

And I don't think that's possible, sadly.

j.