maintenance: use random minute in Windows scheduler

maintenance: use random minute in Windows 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 Windows scheduler integration.

We need only to modify the minute value for the 'StartBoundary' tag
across the three schedules.

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