title: Git vs SVN How to diff against tags
body: <p>With svn and Cornerstone this is trivial, but I can't seem to find a way to do it with Git or sourcetree (Atlassians excellent visual git tool for mac sourcetreeapp).</p>
  
  <p>I need to compare two commits on the same branch that are several versions apart.  Cornerstone lets me pick any version or tag and compare against my working directory but I can't seem to find a way to get sourcetree, or with git diff and filemerge (opendiff) to do this.</p>
  
  <p>E.g.  </p>
  
  <pre><code>working dir HEAD
  change three
  change two
  change one Tag: works
  </code></pre>
  
  <p>I need to compare <code>HEAD</code> and <code>works</code> but I can't seem to find a way to do this.  I can't even check out a file from <code>works</code> like I can in SVN, it wants to pull the whole repository at that version and wipe out my working directory.  If I could get the files side by side I could run opendiff or another tool.</p>
  
  <p>What am I missing?</p>