Parts of "git maintenance" to ease writing crontab entries (and
other scheduling system configuration) for it.

* ds/maintenance-part-3:
  maintenance: add troubleshooting guide to docs
  maintenance: use 'incremental' strategy by default
  maintenance: create maintenance.strategy config
  maintenance: add start/stop subcommands
  maintenance: add [un]register subcommands
  for-each-repo: run subcommands on configured repos
  maintenance: add --schedule option and config
  maintenance: optionally skip --auto process