git diff --submodule=diff and --stat/--dirstat/--name-only

Hello all,

First, thank you to the community for the great work. Worth saying it from time to time, I think.

I am dealing with a couple of big repositories that use git submodules. They have nested submodules, and some of them are pretty huge. I came across:

git diff --submodule=diff

Which is very handy for creating some tooling, but it's obviously slow
- in the order of tens of minutes in my case - for big diffs. I was
only interested in the list files that changed, in this particular
case, but:

git diff --stat --submodule=diff

doesn't seem to honour `--submodule=diff` and it doesn't go into the submodule(s) nor recurses, of course. Other options like `--dirstat` or `--name-only` seem to behave the same way.

I've tried this v2.20.1 and the HEAD of master (d4a392452e) with the same results. Is this a missing feature, a bug or is it just the intended behaviour?

Regards marc