title: How do I update my git submodules from specific branches?
body: <p>I have a Ruby on Rails app that has two git submodules. I am upgrading Rails from v3 to v4 in the main app and in the two submodules, so I created a rails4 branch for each in which to do the work. The main app and each submodule have their own test suites that run independently, so I made the upgrades in the submodules and the project, ensured the tests all run in each place, and now I need to update the submodules into the project.</p>
  
  <p>How do I tell git in my app's rails4 branch to update from the submodules' rails4 branches when I do <code>git submodule update</code>? It's ok to have a temporary fix for this as I can point them back at master once the app is confirmed working and the branches all get merged in to master. Thanks.</p>