title: Git won't fast-forward merge even though the two branches should be in sync
body: <p>I have two branches, <code>dev</code> and <code>next</code>, where <code>next</code> was branched off <code>dev</code>. The branches diverged a little so when I made a hotfix to <code>dev</code> I merged the updates into <code>next</code> with a merge commit as expected. The latest commits on the branches looked like:</p>
  
  <pre><code>// dev
  123455 Latest dev commit
  123454 Earlier dev commits...
  
  // next
  123456 Merge branch 'dev' into next
  123455 Latest dev commit
  123454 Earlier dev and next commits...
  </code></pre>
  
  <p>Now I've made one more commit to <code>dev</code>, but when I try to merge it into <code>next</code> with <code>git merge --ff-only dev</code> it says a fast-forward is not possible. How can this be the case, when the branches were just in sync?</p>