title: Why does `.git/index` change when I haven't done anything to my repository?
body: <p>With the latest Debian version of git (I'm using 1.7.2.5), I've noticed that a <code>.git/index</code> file may change mysteriously, without my having performed any operation that I feel should change the repository.  (My shell occasionally runs <code>git branch</code> so it can display what branch is checked out, but that shouldn't change anything.)   The change results in a <code>.git/index</code> file with the same length as the original, but containing different bits.   <strong>What causes this change, and how can I stop it?</strong></p>
  
  <p>(The change is inconvenient because it messes things up for the <a href="http://www.cis.upenn.edu/~bcpierce/unison/" rel="noreferrer">Unison</a> file synchronizer.)</p>