title: heroku doesn't update submodules after push
body: <p>I have an app hosted on heroku contaiting a submodule. The submodule contains multiple submodules. The submodules are in form of http address to github repo.</p>
  
  <pre><code>app
   |- submodule1
   |- other dirs
  
  submodule1
   |- submodule2
   |- submodule3
   |- other dirs
  </code></pre>
  
  <p>I made changes in several sobmodules then commited everything (including submodules) and pushed to github. I can verify, that submodules point to the correct commits i.e. a repo may be obtained via <code>git clone --recursive ...</code>.</p>
  
  <p>After I pushed to github, I pushed to heroku. The app itself updated, nevertheless the submodules remained the same (even though they are commited and pushed to github)!</p>
  
  <p>Example of <code>.gitmodules</code>:</p>
  
  <pre><code>[submodule "src/main/java/runtime"]
      path = src/main/java/runtime
      url = https://github.com/USER/REPO.git
  </code></pre>
  
  <p>What should I do? This is serious problem for me.</p>