Fork a subdirectory of an upstream repo

fork_subdirectory.sh

# clone an upstream repo
git clone https://github.com/<user>/<project>.git && cd project
git branch -m upstream-master
git subtree split --prefix=<folder>/<subfolder> -b upstream-rebase
git checkout upstream-rebase

git remote rename origin upstream
git remote add origin https://github.com/<my-account>/<my-project>.git
git fetch origin
git push -u origin upstream-rebase

git checkout -b master
git push -u origin master

# To merge upstream changes
git checkout upstream-master
git pull
git subtree split --prefix=<folder>/<subfolder> --onto upstream-rebase -b upstream-rebase