title: How to checkout a tag with GitPython
body: <p>In a python script, I try to checkout a tag after cloning a git repository.
  I use GitPython 0.3.2.</p>
  
  <pre><code>#!/usr/bin/env python
  import git
  g = git.Git()
  g.clone("user@host:repos")
  g = git.Git(repos)
  g.execute(["git", "checkout", "tag_name"])
  </code></pre>
  
  <p>With this code I have an error:</p>
  
  <pre><code>g.execute(["git", "checkout", "tag_name"])
  File "/usr/lib/python2.6/site-packages/git/cmd.py", line 377, in execute
  raise GitCommandError(command, status, stderr_value)
  GitCommandError: 'git checkout tag_name' returned exit status 1: error: pathspec 'tag_name' did not match any file(s) known to git.
  </code></pre>
  
  <p>If I replace the tag name with a branch name, I have no problem.
  I didn't find informations in GitPython documentation.
  And if I try to checkout the same tag in a shell, I have non problem.</p>
  
  <p>Do you know how can I checkout a git tag in python ?</p>