title: Cutting off master branch history and start from "nothing" in git
body: <p>we absorb a project from another company and it's git repo. We only have this one repo and also want to keep the history of the old project.</p>
  
  <p>We started developing in a new branch and now I want to have the master branch "cleaned" and cut off.</p>
  
  <p>Is there a workflow especially for that or is it enough to make a merge? But I think, that if I just merge, some files will be left.</p>