Create a temporary branch and checkout:
$ git checkout --orphan temp_branch
| Option | Description |
|---|---|
| --orphan | Create a branch in a git init-like state |
Add all files to the temporary branch and commit the changes:
$ git add -A
$ git commit -am "The first commit"
Delete the master branch:
$ git branch -D master
Rename the temporary branch to master:
$ git branch -m master
Forcefully update the remote repository:
$ git push -f origin master
see also https://www.shellhacks.com/git-remove-all-commits-clear-git-history-local-remote/