For some reason, presubmit_support.py can't find PRESUBMIT.py
unless it has a diff. Calling this script directly allows us
to specify a fake upstream to force it to find a diff and
therefore be able to find PRESUBMIT.py.
BUG=skia:5823
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2398653002
Review-Url: https://codereview.chromium.org/2398653002