git difftool: how to measure progress, that is number of files diff'ed and to diff?

I know I can use git difftool with the --dir-diff option in order to compare all the files in one go.

But using difftool with prompt has its use (for a reasonably small number of files to diff), especially since git 1.7.8, where you can skip a file.

However, that prompt doesn't show how many files are in the diff queue or how many has already been diff'ed.
How would you display that information in the difftool prompt?