maintenance: swap method locations

maintenance: swap method locations

The systemd_timer_write_unit_templates() method writes a single template
that is then used to start the hourly, daily, and weekly schedules with
systemd.

However, in order to schedule systemd maintenance on a given minute,
these templates need to be replaced with specific schedules for each of
these jobs.

Before modifying the schedules, move the writing method above the
systemd_timer_enable_unit() method, so we can write a specific schedule
for each unit.

The diff is computed smaller by showing systemd_timer_enable_unit() and
systemd_timer_delete_units()  move instead of
systemd_timer_write_unit_templates() and
systemd_timer_delete_unit_templates().

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