With these steps you will remove the complete history from your GIT repository.

So please make a backup first!

# get last version from your repository and create an orphan branch
# Create a new orphan branch, named <new_branch>, started from <start_point> and switch to it. The first commit made on this new branch will have no parents and it will be the root of a new history totally disconnected from all the other branches and commits.
git checkout --orphan latest_branch
# Add everything to this branch
git add -A
# Commit to this branch
git commit -am "Init"
# Delete master branch
git branch -D master
# Move current branch to master
git branch -m master
# Push everything to your remote
git push -f origin master