bug: git difftool does not respect submodule argument

This repository is set up to reproduce the issue:
git clone --recurse-submodules https://github.com/Chetic/super.git

The following command behaves as expected and produces a diff to files
in submodules:
git diff --submodule=diff HEAD~1 HEAD

But the following command produces a diff of commit IDs referred to by
the super repository, i.e. same behavior as without the
"--submodule=diff" argument. I expected this to behave like the
command above but displayed with an external diff tool:
git difftool --submodule=diff HEAD~1 HEAD