title: Two git repos accidentally merged, how to clean up
body: <p>A client of mine accidentally set a different project as the remote in his <code>.git/config</code> file, then pulled, committed and pushed.</p>
  
  <p>This means all of the files from repo "A" (the primary one) now are in repo "B" (the secondary one). Unfortunately, several people have already pulled from "B".</p>
  
  <p>What is the best way to remove all the files, commits and history of "A" from "B"?</p>