A "git gc"'s big brother has been introduced to take care of more
repository maintenance tasks, not limited to the object database
cleaning.
* ds/maintenance-part-1:
maintenance: add trace2 regions for task execution
maintenance: add auto condition for commit-graph task
maintenance: use pointers to check --auto
maintenance: create maintenance.<task>.enabled config
maintenance: take a lock on the objects directory
maintenance: add --task option
maintenance: add commit-graph task
maintenance: initialize task array
maintenance: replace run_auto_gc()
maintenance: add --quiet option
maintenance: create basic maintenance runner