The first attempt didn't migrate all rows on GitLab.com, due to a couple of
issues:
1. Some rows in merge_request_diffs had truly huge numbers of commits and diffs
serialised - one in particular had 26,000 commits!
2. The jobs were sometimes on Sidekiq hosts with frequent OOM errors, leading to
the job being lost.
The previous commit adds more logging, and a more robust insertion method. This
commit reschedules the jobs, with a generous pause between each.
