title: Adding git subtree from a branch
body: <p>I'm trying to add a repo (called cow) to my project using <code>git subtree add</code>.  In particular, I'd like to add the branch <code>stable</code> (which is not the <code>master</code> branch).  I tried:</p>
  
  <pre><code>git subtree add -P cow https://github.com/geoffryan/cow.git stable
  </code></pre>
  
  <p>But this returned the error</p>
  
  <pre><code>'stable' does not refer to a commit.
  </code></pre>
  
  <p>I also tried:</p>
  
  <pre><code>git subtree add -P cow https://github.com/geoffryan/cow.git cow/stable
  'cow/stable' does not refer to a commit.
  </code></pre>
  
  <p>And:</p>
  
  <pre><code>git subtree add -P cow https://github.com/geoffryan/cow.git ca26d248a12c21264e32a2c212381cafb578c9fb
  'ca26d248a12c21264e32a2c212381cafb578c9fb' does not refer to a commit.
  </code></pre>
  
  <p>The hash was that for the latest commit in the <code>stable</code> branch.  The examples of use I've seen online all use <code>master</code> for the commit, is it possible to use <code>subtree add</code> on a non-master branch?</p>