How to merge Git repos with common code BUT NO common ancestry

git_rebase_two_repos.txt

git init
git remote add new https://github.com/patrickwolf/sandbox_repoNew
git remote add old https://github.com/patrickwolf/sandbox_repoRoot
git fetch new
git fetch old
echo "first" > readme.md
git add readme.md
git commit -m "first"
git branch combined
git rebase --rebase-merges --onto 9086ffd 80ece84 combined