title: git subtree pull rebase error
body: <p>I added one repo as subtree, when I run <code>git pull -s subtree projectB master</code>, everything is as expected, but get fatal when switch to <code>rebase merge</code> with <code>git pull --rebase -s subtree projectB master</code>. Log:</p>
  
  <pre><code>First, rewinding head to replay your work on top of it...
  fatal: Could not parse object 'd2c2b2d5548fab404e1fccfa1fc22da3d5a6151c^'
  Unknown exit code (128) from command: git-merge-subtree d2c2b2d5548fab404e1fccfa1fc22da3d5a6151c^ -- HEAD d2c2b2d5548fab404e1fccfa1fc22da3d5a6151c
  </code></pre>
  
  <p>Any ideas?</p>