maintenance: create maintenance.<task>.enabled config

[git] / Documentation / config / maintenance.txt

maintenance.<task>.enabled::
        This boolean config option controls whether the maintenance task
        with name `<task>` is run when no `--task` option is specified to
        `git maintenance run`. These config values are ignored if a
        `--task` option exists. By default, only `maintenance.gc.enabled`
        is true.