A secondary site will now forward project repo pulls to the primary site
if the latest repo change is not yet replicated.

Current limitations:

We assume last_repository_updated_at is a timestamp of the latest
change. So we compare the timestamp of the latest sync to
last_repository_updated_at.

- Unfortunately, project wiki updates also touch
  last_repository_updated_at. So wiki updates will also bump the repo to
  out-of-date.
- Unfortunately, successive last_repository_updated_at touches are
  throttled within Event::REPOSITORY_UPDATED_AT_INTERVAL minutes. This
  is currently set to 5 minutes. So for example, secondary sites will
  think highly active repos are up-to-date after the first sync of each
  5 minute interval.

This is a rough improvement over the current code.

Next, we can iteratively improve the accuracy of
`.repository_out_of_date?` to fully address the underlying issue. It may
be rather involved, so this MR helps by reducing the scope of that
future change.