title: git: squash/fixup earlier commit
body: <p>Suppose you have:</p>
  
  <pre><code>A-B-C
  </code></pre>
  
  <p>Now your build/test fails. The fix should be merged in A. 
  My current work-flow is like this:</p>
  
  <pre><code>$ git commit -m "fixA"
  
  A-B-C-fixA
  
  $ git rebase -i A~1
  </code></pre>
  
  <p>And squash fixA in A, result in: </p>
  
  <pre><code>A'-B-C
  </code></pre>
  
  <p>Is there a command to do something like:</p>
  
  <pre><code>A-B-C + (index with fix for A)
  
  $ git commit -supperdupper A 
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>A'-B-C
  </code></pre>