Feat. req.: add a flag to `git clean` to also remove ignored nested repositories

Currently, running `git clean -dxf` on a repository that includes
another repository under an ignored path would skip said repository:

$ git clean -dxf
Removing foo
Skipping repository ignored-path/repo
Removing bar

This is to request the addition of a new flag to allow altering this
behaviour, i.e. to make `git clean` remove those repositories too.

For me, this feature is relevant for building `*-git` packages from
the AUR, for example https://aur.archlinux.org/packages/paru-git. By
default `makepkg` would clone the source repo into `./src/NAME`, which
creates the aforementioned condition. Without such an option on `git
clean`, cleanup after build is rather complicated.

Thanks and regards,
Simon Cheng