title: Git equivalent of subversion's $URL$ keyword expansion
body: <p>I am considering migrating from subversion to git. One of the things we use subversion for our sysadmins to manage things like configuration files. To that end, we put <code>$URL$</code> into each file, which expands to the file's location in the subversion tree. This lets the admins look at a file on some arbitrary host and figure out where in the tree it came from. </p>
  
  <p>The closest analog I could find is gitattributes. There is the <code>filter=</code> directive, but it seems that git doesn't communicate to the filter what filename it's filtering, which would be necessary to turn <code>$URL$</code> into a path. </p>
  
  <p>There is also the <code>ident</code> directive, which would turn <code>$Id$</code> into the blob hash. This might be usable if one could map that back into a pathname, but my git-fu isn't strong enough. </p>
  
  <p>Any suggestions?</p>
  
  <p>The workflow is as follows:</p>
  
  <ol>
  <li>Admin commits changes to the VCS repo </li>
  <li>Admin updates a central location that has checked out the repo </li>
  <li>Admin pulls the changes to the host using cfengine</li>
  </ol>