title: Submodule at different branch
body: <p>I want to place submodule_A at master and place submodule_B at new_branch.</p>
  
  <p>But, on the master, I can see both submodule_A and submodule_B in the repo.</p>
  
  <p>How to separate the two submodules at the right branch?</p>
  
  <p>Thanks for reply!!!</p>
  
  <p>NOTE:I use the following command before.</p>
  
  <p><code>git submodule add -b new_branch submodule_B;</code></p>
  
  <p>But submodule_B is still existing on master branch.</p>
  
  <p>When I checkout master, the git displays</p>
  
  <p><code>warning: unable to rmdir submodule_B</code></p>
  
  <p>When I checkout new_branch, the git displays</p>
  
  <p><code>warning: unable to rmdir submodule_A</code></p>
  
  <p>==============================================</p>
  
  <p>For Example</p>
  
  <p><a href="https://github.com/ShawnHuang/.vim/tree/master/bundle" rel="nofollow">https://github.com/ShawnHuang/.vim/tree/master/bundle</a></p>
  
  <p>All plugins is submodules at my repo.</p>
  
  <p>On branch master, I want to use ultisnips.</p>
  
  <p>And, on branch junk/snipmate, I want to use snipmate.</p>
  
  <p>I can separate the two submodules at differnt branches at github.</p>
  
  <p>But I can't do it at local.</p>