$ du -sh .git
121M    .git
$ # Removing the origin just to make sure no reference is kept.
$ git remote rm origin
$ echo 21581f18dc98b69c0dad8863d70a0ce7842ce4fe > .git/info/grafts
$ git filter-branch -- --all
Rewrite 4352ea1fb4b0dfa5d28ad26ce6b9e042b126bd60 (8/8)
Ref 'refs/heads/master' was rewritten
$ rm .git/info/grafts
$ du -sh .git
121M    .git