title: how to remove a file from a git commit that has been pushed
body: <p>I mistakenly used 
      git commit -a 
  and added more files than I wanted to in a commit. There was another modified file that I wanted to add as a separate commit included in this commit. How do I go back ( without losing the modifications to the files ) and then commit them all again separately. </p>
  
  <p>I've also pushed this to a remote origin ( though I know for a fact that no one has pulled anything since I pushed). </p>
  
  <p>Many Thanks!</p>