title: What does it mean in the changelog of msysgit 1.7.11 when it says: An experimental "version 4" format of the index file
body: <p>As stated in the title,</p>
  
  <p>what do they mean:</p>
  
  <blockquote>
    <p>An experimental "version 4" format of the index file has been introduced to reduce on-disk footprint and I/O overhead</p>
  </blockquote>
  
  <p>More importantly, is there a risk in losing backwards compatibility with this change, or could this change corrupt the repository?</p>
  
  <p>Doing some testing suggests that it's backwards compatible and did not reveal any adverse effects.</p>
  
  <p>Could somebody please clarify what this change actually is to msysgit 1.7.11?</p>