title: git: update HEAD after fetch into a bare repo
body: <p>I have a bare repo at, say, /var/lib/repos/myrepo.git, I try to update it by doing:</p>
  
  <pre><code>$ cd /var/lib/repos/myrepo.git
  $ git fetch
  </code></pre>
  
  <p>I get the latest commits from remote, however, the HEAD pointer is NOT updated.  If I do</p>
  
  <pre><code>$ git log
  </code></pre>
  
  <p>it just show me the log message BEFORE the update, if I do</p>
  
  <pre><code>$ git log FETCH_HEAD
  </code></pre>
  
  <p>I can see the latest commits.</p>
  
  <p>I want to either update HEAD to point to the sha1 of FETCH_HEAD, or to let me clone the bare repo to another directory at FETCH_HEAD.   However, the FETCH_HEAD reference is NOT available in the working directory.</p>