title: git revision number in source code documentation
body: <p>I've seen in several projects that developers include something like <code>$Revision$</code> in source code documentation. I think they are macros or environment variables for some tool but I do not know how to work with them.</p>
  
  <p>I am interested on adding this feature to my next project. I normally use TextMate, git and doxygen. Any suggestions?</p>