title: how to use vimdiff in VIM command mode?
body: <p>I want to use <code>:vimdiff file1 file2</code> to diff these files in VIM command mode , but I got a error message <code>E492: Not an editor command: vimdiff</code> . do I forget something ? how to resolve it?</p>
  
  <p>According Vim manual, vimdiff command should be available</p>