title: Purging file from Git repo failed, unable to create new backup
body: <p>I tried to remove a file from my remote repo by running:</p>
  
  <pre><code>git filter-branch --index-filter 'git rm --cached --ignore-unmatch Rakefile' HEAD
  </code></pre>
  
  <p>But Git complains that </p>
  
  <blockquote>
    <p>Cannot create new backup. A previous backup already exists in refs/original/<br>
    Force overwriting the backup with -f<br>
    rm: cannot remove /.git-rewrite/backup-refs : Permission denied<br>
    rm: cannot remove directory /.git-rewrite : Directory not empty</p>
  </blockquote>
  
  <p>This was after I already deleted the .git-rewrite directory on Windows.</p>
  
  <p>How can I remove that file? It's a 29Mb file sitting on my repo, so I quite need to remove the file. </p>
  
  <p>I tried to delete the commit in <code>git rebase -i</code>, but apparently because the commit touched a lot of different files, Git complains of conflicts and I aborted to be safe.</p>