maintenance: update schedule before config

maintenance: update schedule before config

When running 'git maintenance start', the current pattern is to
configure global config settings to enable maintenance on the current
repository and set 'maintenance.auto' to false and _then_ to set up the
schedule with the system scheduler.

This has a problematic error condition: if the scheduler fails to
initialize, the repository still will not use automatic maintenance due
to the 'maintenance.auto' setting.

Fix this gap by swapping the order of operations. If Git fails to
initialize maintenance, then the config changes should never happen.

Reported-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>