title: Is it possible to overlay files in a Git submodule?
body: <p>A Git repository <code>A</code> contains another repository as a submodule <code>B</code>.</p>
  
  <p>Can I add a file within the <code>B</code> subtree but commit it only to <code>A</code> ?</p>
  
  <p>The requirement calls for a file that is not included in <code>B</code> to be stored in the <code>B</code> subtree within <code>A</code>. The file should be ignored when committing <code>B</code> but included when committing <code>A</code>.</p>
  
  <p>An extension of the idea would see two versions of the file, an original in <code>B</code> and a customised version in <code>A</code>.  If such a <code>B</code> was cloned on its own then that copy would see the original file but within the submodule you would get the customised version from <code>A</code>.</p>
  
  <p>Is this possible at all ?</p>