Add option to ignore submodules

We have many submodules in our repo, and different build targets require different submodules. Our build process takes care of obtaining the correct submodules depending on what's being built.

The git plugin's default practice of cloning all the submodules in the repo is inefficient for us (many of our submodules are full Linux kernels, for example).

It would be great if there were an option to ignore submodules in the repo.