fbsync should autoclose when a PR is landed/committed

[samdow]

Currently, we're seeing that even after PRs are landed internally (and have a related commit added to the fbsync branch, the original PR stays open, example). This is confusing for us if we aren't sure what has actually synced and what is having troubles landing (internally or externally)

Somewhat related to #796 but should be done sooner as we discuss the right path forward for that