title: How to exclude a specific git submodule from update?
body: <p>I have list of submodules in .gitmodules.
  I want to download a specific submodule i.e grpc only if there is some option enabled as true in config file. 
  Since grpc is not required at times for my build.
  All submodules are in third-party directory.
  So .gitmodules is like:</p>
  
  <pre><code>[submodule "third-party/libzip"]
          path = third-party/libzip
          url = https://github.com/nih-at/libzip.git
  [submodule "third-party/sqlite"]
      path = third-party/sqlite
      url = https://github.com/mackyle/sqlite.git
      branch = sqlite-3.23.1
  [submodule "third-party/grpc"]
      path = third-party/grpc
      url = https://github.com/grpc/grpc.git
  </code></pre>
  
  <p>Also is there a way to exclude the submodule specifically while executing command: </p>
  
  <pre><code>git submodule update --init --recursive
  </code></pre>
  
  <p>I would like to exclude grpc and submodules in grpc while submodule update. Something like:</p>
  
  <pre><code>git submodule update --init --recursive "exclude third-party/grpc"
  </code></pre>