# Example
# Add a new repository as a subtree
git subtree add --prefix=<prefix> <remote-name> <branch> --squash

# it is updated from the main repository
# push to subtree repository

git subtree push --prefix=src/subtree subtree(repo name) master(remote)

# update from subtree repository
git subtree pull --prefix=src/subtree subtree(repo name) master(remote) --squash