title: git cat-file using treeish notation
body: <p>When using the command git cat-file as following:</p>
  
  <pre><code>git cat-file -p master^^{tree}
  </code></pre>
  
  <p>It works well and shows the content of the tree object, 2 commits in the past.</p>
  
  <p>However, when using the same command, changing the ^^ for ~2, it shows the following error:</p>
  
  <pre><code>git cat-file -p master~2{tree}
  
  fatal: Not a valid object name master~2{tree}
  </code></pre>
  
  <p>Is this syntax allowed? Otherwise, how could I retrieve the tree object 100 commits in the past?</p>