title: git diff file against its last change
body: <p>Is it possible to get git to produce a diff between a specific file as it exists now, and as it existed before the last commit that changed it?</p>
  
  <p>That is, if we know:</p>
  
  <pre><code>$ git log --oneline myfile
  123abc Fix some stuff
  456def Frobble the foos
  789dba Initial commit
  </code></pre>
  
  <p>Then <code>git diff 456def myfile</code> shows the last change to myfile. Is is possible to do the same without the knowledge produced by the <code>git log</code>; what changed in 123abc? </p>