Improve GitHub import resume mechanism

At the moment we skip MRs if:

next unless merge_request.new_record? && pull_request.valid?

  1. We should also check if the merge_request is still valid?.

  2. By fetching all PR refs at once as branches we can also skip fetching and recreating the branch.

  3. We should check for new comments/edits too.

Edited by James Lopez