title: Excluding files from git-diff
body: <p>I am tracking a project with git. There are some Xcode project files in the working copy that I want to keep tracking, but do not want to see in diffs, because there are always dozens of changed lines that I am never interested in. Is there a simple way to have <code>git-diff</code> skip these files? I’ve tried to set up a custom “silent” diff tool:</p>
  
  <pre>
  $ cat .gitattributes 
  Project.xcodeproj/* diff=nodiff
  
  $ cat ~/.gitconfig 
  [diff "nodiff"]
      command = /bin/true
  </pre>
  
  <p>But:</p>
  
  <pre>
  $ git diff
  external diff died, stopping at Project.xcodeproj/zoul.mode1v3.
  </pre>
  
  <p>What am I doing wrong?</p>