title: git diff with opendiff gives "Couldn't launch FileMerge" error
body: <p>I have git configured to use ~/bin/opendiff-git.sh as my external diff tool. That script looks like this:</p>
  
  <pre><code>opendiff $2 $5
  </code></pre>
  
  <p>When I try and do a git diff from the command line, I get this message:</p>
  
  <pre><code>2011-02-18 13:58:55.532 opendiff[27959:60f] exception raised trying to run FileMerge: launch path not accessible
  2011-02-18 13:58:55.535 opendiff[27959:60f] Couldn't launch FileMerge
  external diff died, stopping at source/some_file.m.
  </code></pre>
  
  <p>What's going on? This has worked for many months, but stopped working recently.</p>