title: git fast-import --export-marks flag
body: <p>git fast-import --export-marks is able to export a file associating the marks with the commit hashes it created.</p>
  
  <p>So far I've seen the marks are not the ones provided in the input but some "internal ones" not related with the input.</p>
  
  <p>Wouldn't it be much better, for import/export interop, if it keeps the original marks?</p>