title: Git Directory Diff does not work in my submodule
body: <p>I am using Git 1.8.4 and Linux (Kubuntu 13.10). </p>
  
  <p>I have a Git repository with submodules. If I ask for directory diff in the main Git module, it works fine. If I do the same in one of the submodules, I get an error:</p>
  
  <pre><code>~/Projects/MAINMODULE/platform/SUBMODULE [master]$ git difftool -tool=meld --dir-diff --cached
    fatal: Could not switch to '../../../../platform/': No such file or directory
    diff --raw --no-abbrev -z --cached: command returned error: 128
  ~/Projects/MAINMODULE/platform/SUBMODULE [master]$ cd ..
  ~/Projects/MAINMODULE/platform [master]$ cd ..
  ~/Projects/MAINMODULE [master]$ git difftool -tool=meld --dir-diff --cached
    // NO PROBLEM, works.
  ~/Projects/MAINMODULE [master]$ git version
    git version 1.8.4
  </code></pre>
  
  <p>Do you have any idea? May it be a Git Bug?</p>
  
  <p>UPDATE: 1.8.5.3 produces the same output</p>