[feature] git clean ignored files in nested repositories support

Hi,

I am sure this was discussed in the past, I could not find any explicit discussion.

The `git clean` is a handy command, it can clean directories, including ignore files and also handle nested git repositories if a second -f is given.

    git clean -dxff

However, as far as I understand, it is not possible to clean ignored files in nested repositories, the `x` is applied only to the local repository.

As workaround following command may be used:

    git clean -dxff && git submodule foreach git clean -dxff

However, I expect the double `f` to take into account the `x` and also clean ignored files from the nested repository.

I am unsure if this is a bug or intentional.

If this is intentional, maybe a patch to have a second -x would be acceptable? so that we could have something like:

   git clean -dxxff

Any other suggestions? It would be nice to be able to clean the entire tree with a simple git clean command consistently in all scenarios.

Regards, Alon