title: git: retroactively mark a deleted and new file as moved instead
body: <p>I've got a few files that I moved from a root folder into a <code>lib/</code> folder.  I've also made some significant changes to those files as well.</p>
  
  <p>The git history shows the old file as deleted and the new one as added.  But even though there are a lot of differences in the files, I still want them to be considered the same file.  Is it possible to go back and tell git that this file is a move, as opposed to a delete and add?</p>