Incremental backup skips some new events

[behrisch]

Our project https://github.com/eclipse/sumo/ has some large meta tickets like eclipse-sumo/sumo#21 where the backup does not pickup the new events anymore. When looking at the log, the issue does not even show up in the list of updated issues. This might be an upstream problem, at least I found no clear definition of what the "since" parameter in the query considers as an update and there is at least one report at https://github.community/t/github-api-how-to-get-issues-since-create-time/13680 which talks about a similar problem.