How to reset a git repository to the initial state without deleting data

# save backup if necessary

# get initial commit sha-id
$ git rev-list --max-parents=0 HEAD
# <initial-commit-sha-id>

# change the state to the initial commit
$ git reset --soft <initial-commit-sha-id>

# amend the initial commit
$ git commit --amend -m "Initial commit"

# sync the remote
$ git push -f