Always validate new YAML documents on PRs
    
    Previously, if the YAML was entirely new, it was being ignored
    because we were switching to the master branch to run the tests.
    
    Now we will check the master branch out in a temporary location
    to run its tests against the current PR.
    
    Also, if the current PR/commit is a descendent of origin/master,
    it will use the tests from this commit. This should make it easier
    to submit PRs for the tests.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>