# Get a clean branch
git checkout -b new-addon-branch
git reset <root commit sha> --hard

# Apply the patch on the branch
git apply <patch>

# Check the diff
git diff master..new-addon-branch

# And may try to rebase (will work good if your code isn't too bloated with other addon)
git rebase master

# Then, you merge
git checkout master
git merge new-addon-branch