title: git fetch vs. git fetch origin master have different effects on tracking branch
body: <p>This is mostly of the nature of a curiosity as I'm trying to get familiar with Git. I have looked at the documentation for 'git fetch' but I don't see an obvious explanation for the below.  Thanks in advance, and apologies if this is howlingly obvious.</p>
  
  <p>1) From a central repository, say GitHub, I clone a repository named <code>website</code> on each of two machines, <code>HostA</code> and <code>HostB</code>.  </p>
  
  <p>2) on <code>HostA</code>, I make a change to a file, say <code>README.txt</code>, and commit it.<br>
  At this point on <code>HostA</code>, the commits for branches <code>master</code> and
  <code>origin/master</code> are, as expected different since I haven't pushed yet</p>
  
  <pre><code>git show master
  git show origin/master
  </code></pre>
  
  <p>report different hashes (since <code>master</code> has the change and <code>origin/master</code> does not)</p>
  
  <p>3) Once I push, they are after that the same.</p>
  
  <hr>
  
  <p>4) Now, over on <code>HostB</code>, if I do the following:</p>
  
  <pre><code>git fetch
  git merge FETCH_HEAD
  </code></pre>
  
  <p>afterwards, on HostB <code>master</code> and <code>origin/master</code> report the same hash when queried with <code>git show</code></p>
  
  <p>BUT</p>
  
  <p>if instead I had done, on <code>HostB</code>:</p>
  
  <pre><code>git fetch origin master
  git merge FETCH_HEAD
  </code></pre>
  
  <p>at that point the hashes still differ.</p>
  
  <pre><code>git show origin
  git show origin/master
  </code></pre>
  
  <p>report <em>different</em> hashes</p>
  
  <p>The tracking branch <code>origin/master</code> isn't updated until I do a plain <code>git fetch</code></p>
  
  <p>Why is this?</p>