title: Please enter a commit message to explain why this merge is necessary, especially if it merges an updated upstream into a topic branch
body: <p>I am using Git. I did a pull from a remote repo and got an error message:</p>
  
  <blockquote>
    <p>Please enter a commit message to explain why this merge is necessary,<br>
    especially if it merges an updated upstream into a topic branch.  </p>
  </blockquote>
  
  <p>I try to type a message and press <KBD>Enter</KBD> but nothing happens.</p>
  
  <p><strong>How do I tell Git/Terminal I am done typing in my message?</strong></p>
  
  <p>I am using Terminal on OS X.</p>