Fix sense of git diff --quiet return code in prepare_downstream_build

  # This is here because PyOpenCL needs to record a config change so

  # CL headers are found. It git adds siteconf.py.

  if git diff --quiet HEAD; then

  if ! git diff --quiet HEAD; then

    git config --global user.email "inform@tiker.net"

    git config --global user.name "CI runner"

    git commit -a -m "Fake commit to record local changes"