git clone --bare [email protected]:someuser/repo.git
cd repo.git
git push --mirror [email protected]:you/private.git
cd ..
rm -rf repo.git
git remote add upstream [email protected]:someuser/repo.git
git pull upstream master # creates a merge commit
git push origin master
git clone [email protected]:someuser/repo.git
cd repo
git remote add private [email protected]:you/private.git
git checkout -b pr_yourchanges
git pull private master
git push origin pr_yourchanges