title: git merge and got this error "does not point to a commit"
body: <p>I forked a repo from github.com, and also had set the upstream remote.
  Below is my repo</p>
  
  <pre><code>[ Leezhm LIZHM ~/Developments/OF_GIT ] git remote -v
  origin  git@github.com:leezhm/openFrameworks.git (fetch)
  origin  git@github.com:leezhm/openFrameworks.git (push)
  upstream    https://github.com/openframeworks/openFrameworks.git (fetch)
  upstream    https://github.com/openframeworks/openFrameworks.git (push)
  </code></pre>
  
  <p>when I used this command</p>
  
  <pre><code>git fetch upstream develop
  git merge upstream/mac8
  </code></pre>
  
  <p>It always got the following error</p>
  
  <pre><code>fatal: 'upstream/mac8' does not point to a commit
  </code></pre>