Update Git Submodules On Refresh

[ryanjbaxter]

When using a Git repo as a backend and that Git repo contains a submodule you can clone the submodules but they are never refreshed.

See https://github.com/spring-cloud/spring-cloud-config/issues/1917#issuecomment-873216819

I think we can use SubmoduleWalk to walk the submodules and update them https://download.eclipse.org/jgit/site/5.12.0.202106070339-r/apidocs/index.html

[ryanjbaxter]

@spencergibb let me know if you think there is a way to make this work today

[spencergibb]

Not that I'm aware of