title: what is wrong about "error: failed to push some refs to"
body: <p>I cloned a remote branch to local, <br>
  $git clone -b my_branch REMOTE_REPO <br>
  made some changes and then was going to push back to the remote branch. <br>
  $git push origin master:my_branch <br></p>
  
  <p>But the push was rejected:</p>
  
  <pre><code>Counting objects: 30, done.
  Delta compression using up to 4 threads.
  Compressing objects: 100% (12/12), done.
  Writing objects: 100% (16/16), 4.15 KiB, done.
  Total 16 (delta 8), reused 7 (delta 4)
  remote: Resolving deltas: 100% (8/8)
  remote: Processing changes: done
  To ssh://user@gerrit.foo.net:29418/foo.git
   ! [remote rejected] my_branch -&gt; my_branch (non-fast forward)
  error: failed to push some refs to 'ssh://user@gerrit.foo.net:29418/foo.git'
  </code></pre>
  
  <p>So what's wrong?</p>