title: p4merge fails when doing a directory diff?
body: <p>I am trying to use p4merge as my diff/merge tool in git.</p>
  
  <p>But I got a serious problem.</p>
  
  <p>I can use p4merge as a difftool with <code>git difftool &lt;sha1&gt; &lt;sha1&gt;</code> it works well. And I can also use it as a mergetool. But when I try to let it do a directory diff like <code>git difftool -d &lt;sha1&gt; &lt;sha1&gt;</code> it pops up an error dialog which says:</p>
  
  <blockquote>
    <p>Errors:
    '/var/folders/6s/6sCbckgPGH42yLSh2eXveE+++TI/-Tmp-/git-difftool.9rgKV/left'
    is (or points to) an invalid file.
    '/var/folders/6s/6sCbckgPGH42yLSh2eXveE+++TI/-Tmp-/git-difftool.9rgKV/right'
    is (or points to) an invalid file.</p>
    
    <p>Use 'p4merge -h' for more help.</p>
  </blockquote>
  
  <p>So is this caused because p4merge doesn't support directory diff? is there any way to fix this?</p>
  
  <p>BTW, this is my <code>.gitconfig</code> settings</p>
  
  <pre><code>; p4merge settings
  ; Can't do directory diff?
  [difftool "p4merge"]
      cmd = /Applications/p4merge.app/Contents/MacOS/p4merge \"$LOCAL\" \"$REMOTE\"
      trustExitCode = false
  [mergetool "p4merge"]
      cmd = /Applications/p4merge.app/Contents/MacOS/p4merge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\" 
      trustExitCode = false
  
  [difftool]
      prompt = false
  [diff]
      ; tool = kdiff3
      tool = p4merge
  [merge]
      ; tool = kdiff3
      tool = p4merge
  </code></pre>