Prefetch maintenance might lead to thundering herd issues

The current git-maintenance prefetch command allows daily, weekly, and hourly schedules. However, in repositories with many active developers, timing `fetch` to happen at the same wall clock time can lead to a thundering herd problem for the backend server.

Ideally, these fetch requests would be spaced out by a small amount, so as not to lead to a co-ordinated load on the server.

I don't see a way to configure this at the moment in a cross-platform way at the moment. Perhaps it might be sensible to offset the cron schedules slightly for users so as to not cause such an issue?

This might come at a small cost of predictability of the run, but that should be okay as predictability isn't the primary goal for maintenance.