title: Disable composer cache for git clone
body: <p>When composer tries to clone my git repo from Bitbucket, it loads it from cache. Where the latest commits from master are not loaded which results in an outdated repo. Clearing the composer cache each time seems cumbersome. 
  I want to force composer to load this repo never from cache. <code>--prefer-source</code> could do the trick. But how do go about this in the best way. How to add the force no cache in the code below? </p>
  
  <pre><code> "mybbrepo": {
          "type": "package",
          "package": {
              "name": "project/mybbrepo",
              "version": "1.0",
              "type": "drupal-theme-custom",
              "source": {
                  "url": "git@bitbucket.org:project/mybbrepo.git",
                  "type": "git",
                  "reference": "master"
               }
          }
      },
  </code></pre>