Because we're extending the time between when the downstream syncer runs and when its PR gets merged, we need to explore ways to ensure it is more robust to downstream-only changes that can be missed depending on timing of the downstream-only changes and when the syncer is running.

Background
Currently the syncer checks for upstream changes which then triggers a check for downstream carry commits. However, in the case where there are downstream carry commits with no corresponding upstream changes in the same time period, the syncer can miss some downstream commits because it is not explicitly checking for downstream carry commits if no upstream changes are found first. In addition, the syncer doesn't explicitly keep track of where it left off checking downstream and can miss carries that way.

Goal
Identify and implement changes to the syncer to make it more robust to longer times between initial run/merge and to downstream-only carry commits.