title: git add submodule with depth and branch options
body: <p>Using git 2.4.3 (Fedora 22), I can clone a repo using the <code>--depth</code> and <code>-b</code> options:</p>
  
  <pre><code>$ git clone --depth 1 -b release https://github.com/adobe-fonts/source-code-pro.git fonts/source-code-pro
  Cloning into 'fonts/source-code-pro'...                                    
  remote: Counting objects: 114, done.
  remote: Compressing objects: 100% (113/113), done.
  remote: Total 114 (delta 1), reused 105 (delta 1), pack-reused 0
  Receiving objects: 100% (114/114), 7.27 MiB | 2.85 MiB/s, done.
  Resolving deltas: 100% (1/1), done.
  Checking connectivity... done.
  </code></pre>
  
  <p>In theory <code>git submodule add</code> should support the same options. Unfortunately it doesn't seem to work:</p>
  
  <pre><code>$ mkdir foo
  $ cd foo
  $ git init
  Initialized empty Git repository in /spare/local/arankine/foo/.git/
  $ git submodule add --depth 1 -b release https://github.com/adobe-fonts/source-code-pro.git fonts/source-code-pro                        
  Cloning into 'fonts/source-code-pro'...
  remote: Counting objects: 35853, done.
  remote: Compressing objects: 100% (5932/5932), done.
  remote: Total 35853 (delta 35196), reused 30018 (delta 29921), pack-reused 0
  Receiving objects: 100% (35853/35853), 12.95 MiB | 3.06 MiB/s, done.
  Resolving deltas: 100% (35196/35196), done.
  Checking connectivity... done.
  fatal: Cannot update paths and switch to branch 'release' at the same time.
  Did you intend to checkout 'origin/release' which can not be resolved as commit?
  Unable to checkout submodule 'fonts/source-code-pro'
  </code></pre>
  
  <p>The intention here is to minimize the disk space required for these fonts, which are distributed as binary objects on the <code>release</code> branch of that repo. Only the latest binaries are relevant.</p>
  
  <p>It's not obvious to me how to make this work, please advise.</p>