Adding the subtree

Open bash and navigate to the root of your project. 

Now, add the default skin as a subtree:

git subtree add --prefix theme/www/assets/parentnode-skin-default master --squash

You might encounter an error: "Working tree has modifications. Cannot add." In that case, make sure that you are in sync with your project repository. You must use git pull from bash for this to work. 

Now that you've added a subtree to your project, which git automatically commits, you can push your changes to the repo.

Updating the subtree

Again, open bash and navigate to the root of your project. Here's the command you need:

git subtree pull --prefix theme/www/assets/parentnode-skin-default master --squash