maintenance: use random minute in launchctl scheduler

maintenance: use random minute in launchctl 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.

Use get_random_minute() when constructing the schedules for launchctl.

The format already includes a 'Minute' key which is modified from 0 to
the random minute.

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