Support git submodules

[matfantinel]

So, I am using this Action to build and deploy a Jekyll site to GH Pages. It was working fine (thank you for this, btw!) until I started using a git submodule in my repo.

This submodule links to another repo with icons. The idea of using it as a submodule is so that the website always displays up-to-date icons, without having to re-download them when they get updated.

I noticed that the folder that points to the other git repo is not on the gh-pages branch. When building locally, the folder is successfully moved to /build.

Here is the git repo, the submodule folder is here, and here is the result of the build using this Action.