maintenance: use random minute in cron scheduler

maintenance: use random minute in cron scheduler

The get_random_minute() method was created to allow maintenance
schedules to be fixed to a random minute of the hour. This randomness is
only intended to spread out the load from a number of clients, but each
client should have an hour between each maintenance cycle.

Add this random minute to the cron integration.

The cron schedule specification starts with a minute indicator, which
was previously inserted as the "0" string but now takes the given minute
as an integer parameter.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>