Bug 11831

The current gitsm implementation is naive at best, broken at worst, and lets git itself do the fetching of the submodules, so we can't use our mirror tarballs or shallow tarballs for them, among other issues, such as the already reported issue with license.bbclass.

This issue is to track its replacement with a version that uses the bitbake fetcher to fetch the submodules recursively, and provides the urls of all the submodules to the metadata (likely stored as a member of the fetcher object, so anyone that needs it will have to isinstance() and grab it from there, as we can't get that information at parse time, only after the download()).

We can implement direct logic to do this, or we can try implementing a fetch helper to let git call back to us to do the repository fetch for it.