title: Add git submodule using a specific commit number
body: <p>I'm using git submodule commands to add <a href="https://github.com/tkem/fsmlite" rel="nofollow noreferrer">this FSM repo</a> in my project. I want to checkout a specific release commit. By default the master branch is checked out. 
  After adding the git repo, when I run</p>
  
  <p><code>$ git submodule</code></p>
  
  <p>It gives </p>
  
  <p><code>d1b66d66cfa95f238a7498465908a262f4b2326a directory_path/fsmlite</code></p>
  
  <p>The commit number here belongs to a master branch commit. How can I checkout another commit instead, using its commit number?</p>