title: Gerrit always rebase before submit patchset to avoid a merge commit
body: <p>I found that when clicking "Submit Patch Set" in the <strong>Gerrit</strong> web interface, it'll either simply add a commit to that branch, or create a merge commit if another commit was submitted just before.</p>
  
  <p>Example that creates 2 commit: The actual commit and a merge commit:</p>
  
  <ol>
  <li>User submits patchset A depending on commit O</li>
  <li>User submits patchset B depending on commit O</li>
  <li>Submit Patch Set A</li>
  <li>Submit Patch Set B --> Creates merge commit between O -> A and O -> B</li>
  </ol>
  
  <p>There is a "Rebase Change" button which is great but it means that to submit a patch set everyone should always do:</p>
  
  <ol>
  <li>Click <strong>Rebase Change</strong></li>
  <li>Click <strong>Submit Change Set</strong></li>
  </ol>
  
  <p>The only reason I see a merge commit useful is to keep dates of the commits (but I do understand why it is required without a rebase).</p>
  
  <p>Isn't there an <strong>automatic rebase</strong> or at least a check to avoid generating an undesired merge commit?</p>