title: get short sha of commit with gitpython
body: <p>The long SHA can be gotten like below:</p>
  <pre><code>repo = git.Repo(search_parent_directories=True)
  sha = repo.head.object.hexsha
  </code></pre>
  <p>Or, in git 3.1.7:</p>
  <pre><code>sha = repo.head.commit.hexsha
  </code></pre>
  <p>How about short one?
  (short <code>SHA</code> is decided by the scale of the repo, so it should not be like <code>sha[:7]</code>)</p>