Only checkout specific submodules

[Maddimax]

I would like to be able to specify which submodules I want to check out, instead of being forced to checkout all or nothing.

A Merge request for this has been opened a while ago (#1866), and the comments for it show general interest in the feature.

Please consider it for inclusion.

[dbabokin]

That would be really useful for me. I have a repo with a tiny service submodule and a huuuuge https://github.com/llvm/llvm-project/ submodule. For some of my workflows I need just a service submodule. And llvm-project checkout takes 6-9 minutes.