title: Why does Xcode 4 version editor sometimes say "no editor"
body: <p>In the Project Navigator, there is a little "M" next to the project file, meaning it's been modified.
  So I tap the Version Editor.  The left side shows me the contents of the project file, and the right side says "No Editor".  Same thing if I go into Source Control, Commit Selected Files...</p>
  
  <p>However, other files (.m, .h, .plist, etc.) all show real differences, and correctly check-in changes.</p>
  
  <p>Do I have this mis-configured somehow?</p>