Re: Initial support for cloning submodules

The plumbing part looks very good, although I sensed a slight slop toward the end (will comment on individual patches later).

I do not like the Porcelain part very much, though. I do not think we would want to add anything new to git-clone. We should lose as much code from git-clone that is common with git-fetch as we can first, and add new features to git-fetch, with possibly passthru options added to git-clone as needed (e.g. a new --submodule option).

If you --submodule cloned a remote repository when it had two submodules, and then later the remote adds another submodule, you would need to have a way to fetch that can discover the presense of the new submodule and add it for you, and at that point, having the code that knows much about submodules in clone would not help you much.

I suspect that a possible interaction between git-fetch and git-clone would go like this:

 (1) "git-clone [--submodules]" would perform a normal clone,
     having most of its work done by git-remote and git-fetch;

 (2) when "--submodules" is given to "git-clone", it passes it
     through to underlying "git-fetch";

 (3) "git-fetch --submodules", after finishing what it would do
     without "--submodules" option, would inspect the fetched
     tree (or the index derived from it), find the tree entries
     with mode 160000 (i.e. submodule graft points), and _then_
     uses the pathnames of these tree entries to consult the
     config mechanism to see which URL(s) can be used to
     retrieve them, probably only for new submodules.  Your new
     "config --remote" mechanism may be one good way to prime
     the configuration from the originating site.

We could probably extend the upload-pack protocol to send the same information as you are using submodules.*.url for, instead of adding a yet another protocol and program pairs you used for "git config --remote". Having a generic program and protocol to dump the whole configuration file is certainly simpler, easier to debug, and easier to repurpose, it makes me somewhat worried about security implications (if it is open to http then worrying about it is not very useful, though).