title: Can't push a commit of Git repo with a submodule
body: <p>I've recently added submodule "C" to my repo "B", then checked out a particular commit in submodule C (a previous release of that tool I want to include in B), then committed B and attempted to push. I get this error:</p>
  
  <pre><code>$ git push --recurse-submodules=on-demand
  </code></pre>
  
  <p>...</p>
  
  <pre><code>Everything up-to-date
  The following submodule paths contain changes that can
  not be found on any remote:
    _submodules/C
  
  Please try
  
          git push --recurse-submodules=on-demand
  
  or cd to the path and use
  
          git push
  
  to push them to a remote.
  
  fatal: Aborting.
  fatal: The remote end hung up unexpectedly
  gitpub error:  process git-shell failed with status 128
  </code></pre>
  
  <p>I should mention that the commit in C that I have checked out is parentless/detached (not sure why the developer opted to have it this way). This appears to cause an issue when I want to push my commit in B.  It might also be relevant to mention that B is a submodule of a superproject A as well.</p>
  
  <p>Thanks for any insight you may be able to provide.</p>