title: How do you merge two Git repositories?
body: <p>Consider the following scenario:</p>
  
  <p>I have developed a small experimental project A in its own Git repo. It has now matured, and I'd like A to be part of larger project B, which has its own big repository. I'd now like to add A as a subdirectory of B.</p>
  
  <p>How do I merge A into B, without losing history on any side?</p>