Make a change in a composer vendor folder and push to branch

gistfile1.txt

# Go into the directory of the vendor package that you changed
cd vendor/<owner>/<package>

# Create a new git repository (composer does not check out the full git repo by default. Only a sparse copy)
git init

# Create a new branch for your local changes
git checkout -b branchname

# Commit changes
git add .
git commit -m "Commit message"

# Add the git remote, which contains the source code
git remote add origin ssh://git@<repo>.git

# Pull changes from remote (sync)
git pull origin master --allow-unrelated-histories

# Fix merge any conflicts
# then push your branch
git push --set-upstream origin branchname

# You can now create a Pull Request if you want