prune accidental key committed to repo

gistfile1.txt

$ mkdir repo_cleanup
$ cd repo_cleanup
$ git clone https://github.com/YOUR-USERNAME/YOUR-REPOSITORY .
$ git filter-branch --force --index-filter \
 "git rm --cached --ignore-unmatch key.txt" \
 --prune-empty --tag-name-filter cat -- --all
$ echo "key.txt" >> .gitignore
$ git add .gitignore
$ git commit -m "Add key.txt to .gitignore"
$ git push origin --force --all
$ git push origin --force --tags