title: Require a downstream job to use the same commit as its parent
body: <p>If job "B" is downstream of job "A" and both draw from the same git repository, how can one ensure that the build of B triggered by a build of A uses the same commit, even if new commits have been made to the repository while "A" was building?</p>
  
  <p>The answer to <a href="https://stackoverflow.com/questions/14374621/jenkins-how-can-i-use-just-created-build-in-downstream-job">this post</a> suggests using the copy-artifact plugin, but it seems like there ought to be an easier way to just instruct job "B" to "use this exact commit hash for this build."</p>