Change Presubmit: let pcg know upstream ref when running tryjob.
Change Presubmit: let pcg know upstream ref when running tryjob.

BUG=529936

Review URL: https://codereview.chromium.org/1347823004

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go