title: switching a subdirectory managed by git to a submodule
body: <p>We used to have a local hack of delayed_job in a Rails app, in vendor/plugins/delayed_job.  It was installed as a one-time event and checked into git in the main app repo.</p>
  
  <p>Now we decided to fork delayed_job on github and replace the subdirectory by a git submodule, as described e.g. here:</p>
  
  <p><a href="http://doblock.com/articles/using-git-submodules-to-manage-plugins-in-rails">http://doblock.com/articles/using-git-submodules-to-manage-plugins-in-rails</a></p>
  
  <p>Before doing that, I simply removed vendor/plugins/delayed_job, without checking it in.  Now, despite adding the submodule, git status in the main repo still shows new files in vendor/plugins/delayed_job.</p>
  
  <p>How should we handle the situation where a subdirectory which was a part of the repo is deleted and made to hold a git submodule?  Should we first delete it with git rm, or obliterate it even more thoroughly, before cloning a submodule into its place?</p>