title: Bitbucket / git checkout specific folders when running pipeline
body: <p>I'm working with a set of applications stored in a git repository.  Some code is suitable for unit testing, but quite a bit of it isn't (yet).  </p>
  
  <p>I set up a bitbucket pipeline to build and test the unit-testable code.  That seems to work fine.</p>
  
  <p>I would like to avoid checking out the remainder of the code.  Is there a way to check out only a subset of the branch?</p>
  
  <p>Thanks.</p>