title: Rewrite git history to squash a bunch of old commits
body: <p>I have  a git repository with ~6000 commits and 40 branches, which is a fork of previous repository with ~5000 commits.
  Is it possible to squash all 5000 commits that came from original repo to a single &quot;initial&quot; commit, and rewrite the history of the new repository on top of it for all branches?</p>