This is mostly possible with git, though it is not a typical use case, so there may be some rough edges as you do this (especially if you are new to git).
The tool that we'll use for this job is git subtree.
Setting up a repository
Start by cloning the whole XBMC repository.
git clone https://github.com/xbmc/xbmc.git
cd xbmc
We start on the master branch by default. We want to make our own master branch, so let's rename master to upstream-master.
git branch -m upstream-master
Now use git subtree split to only include the part that you want. We'll make the split off part a new branch called upstream-skin.
git subtree split --prefix=addons/skin.confluence -b upstream-skin
git checkout upstream-skin
This gives you a new upstream-skin branch that only contains the contents of addons/skin.confluence, and with a filtered history that contains only the commits that modified files in addons/skin.confluence.
Now, let's set up our remotes. Since you cloned xbmc/xbmc.git, the origin remote will point there. Let's rename that to upstream.
git remote rename origin upstream
Make a repository on Github to contain your modifications to addons/skin.confluence. As an example, I'll use tbekolay/xbmc-skin, but replace this with your own repo. Add this repo as a remote, and push your upstream-skin branch to it.
git remote add origin https://github.com/tbekolay/xbmc-skin.git
git fetch origin
git push -u origin upstream-skin
Finally, we'll make a new branch called master that will contain your changes.
git checkout -b master
git push -u origin master
You now have a "fork" of the addons/skin.confluence subdirectory.
Making changes to your repositories
When you're dealing with your own local and remote repositories, you can use normal git commands. Make sure to do this on the master branch (or some other branch, if you'd like) and not the upstream-skin branch, which should only ever contain commits from the upstream project.
git checkout master
echo "My XBMC Skin" > README
git add README
git commit -m "Added README"
git push
Receiving upstream commits
When you're dealing with the upstream repository, you will have to use a mix of git and git subtree commands. To get new filtered commits, we need to do it in three stages.
In the first stage, we'll update upstream-master to the current version of the XBMC repository.
git checkout upstream-master
git pull
This should pull down new commits, if there are any.
Next, we will update upstream-skin with the new filtered version of the commits. Since git subtree ensures that commit hashes will be the same, this should be a clean process. Note that you want to run these commands while still on the upstream-master branch.
git subtree split --prefix=addons/skin.confluence \
--onto upstream-skin -b upstream-skin
With upstream-skin now updated, you can update your master branch as you see fit (either by merging or rebasing).
git checkout master
git rebase upstream-skin
Note that the XBMC repository is gigantic, and the git subtree commands will take quite a bit of time to filter through all that history -- and since you're regenerating the split subtree each time you interact with the remote repository, it's quite an expensive operation. I'm not sure if this can be sped up.
This blog post goes into some more detail on the commands above. Also see the git-subtree docs for even more detail.
