How do I restart a git project, and keep its history?

Question

Much like this question, but here I do care about forkers and history.

I restarted a project from scratch, and now I want to append it as a commit to an existing git repository. How can I achieve this?

Answer 1

Delete the .git of the old repo, and in the new git repo:

cd /path/to/new/git/repo
git add --work-tree=/path/to/old/git/repo add -A .