Use good git hygiene and create a new branch to merge the upstream so you can test for breaking changes: git checkout -b upstarter

Add the remote to upstream: git remote add upstream

Fetch the upstream: git fetch upstream

Then when you merge it use the —allow-unrelated-histories argument: git merge upstream/master --allow-unrelated-histories

You’ll most likely have conflicts to resolve, but after that you can merge it back into master:

git checkout master
git merge upstarter