title: Changing an existing submodule's branch
body: <p>When I initially added my submodule, I specified a particular branch, as seen in the <code>.gitmodule</code> file:</p>
  
  <pre><code>[submodule "externals/grpc/grpc"]
      path = externals/grpc/grpc
      url = git@github.com:me/grpc.git
      branch = release-1.0
  </code></pre>
  
  <p>I want to change to the master branch of my submodule, so I changed the branch in <code>.gitmodules</code> from <code>release-1.0</code> to <code>master</code>, and for good measure, just deleted the submodule from my parent git tree:</p>
  
  <pre><code>cd $submodules
  rm -rf grpc
  cd $gitroot
  git submodule sync
  git submodule update --init --recursive
  </code></pre>
  
  <p>Now, when I go back to my submodule, it's still checked out from a commit on the <code>release-1.0</code> branch, not the latest master commit.</p>
  
  <p>What steps am I missing to switch my submodule's branch?</p>