Add support for submodules

[aexvir]

Hi there!
First of all, thanks for developing this plugin 🙂

We developed a common documentation platform that also allows "importing" resources via using git submodules. Right now this plugin doesn't support this as everything it's scoped to the main git repo, so we are missing that data for files fetched from submodules.

Would it be possible to spawn the git processes always in the same folder as the node checked? I think this should address this issue, as that's how git natively behaves.