Repository.Submodules.Add created (v2)

[olmobrutall]

I opened this PR 13 years ago :) #482

I have merged the latest master and make work again, with the functionality moved to Commands.

Here is a usage example:

Commands.AddSubmodule(rep, si.Name, si.Url, si.Path, useGitLink: false,
    initiRepository: subRep =>
    {
        subRep.Network.Remotes.Add("local", Path.Combine(config.SouthwindProjectDirectory, si.Path));
        Commands.Fetch(subRep, "local", new string[0], FetchOptions(si.Name, progress), null);
        Commands.Checkout(subRep, (string)si.IndexCommitId.ToString(), CheckoutOptions(si.Name, progress));
        subRep.Network.Remotes.Remove("local");
        Commands.Fetch(subRep, "origin", new string[0], FetchOptions(si.Name, progress), null);
    });

What would be needed to get this merged?