title: How to initialize git submodule when you already have a local copy?
body: <p>I already have a git repo inside another one. Let's say the directory structure is:</p>
  
  <pre><code>repo1
    --.git
    --repo2
      --.git
  </code></pre>
  
  <p>How would I be able to tell git I want repo2 to be a submodule? Note that I already have the entire git repo on disk (I tried doing <code>git submodule add repo2</code>, but it said <code>repo URL: 'repo2' must be absolute or begin with ./|../</code>)</p>