maintenance: use random minute in systemd scheduler

maintenance: use random minute in systemd 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 systemd integration.

This integration is more complicated than similar changes for other
schedulers because of a neat trick that systemd allows: templating.

The previous implementation generated two template files with names
of the form 'git-maintenance@.(timer|service)'. The '.timer' or
'.service' indicates that this is a template that is picked up when we
later specify '...@<schedule>.timer' or '...@<schedule>.service'. The
'<schedule>' string is then used to insert into the template both the
'OnCalendar' schedule setting and the '--schedule' parameter of the
'git maintenance run' command.

In order to set these schedules to a given minute, we can no longer use
the 'hourly', 'daily', or 'weekly' strings for '<schedule>' and instead
need to abandon the template model for the .timer files. We can still
use templates for the .service files. For this reason, we split these
writes into two methods.

Modify the template with a custom schedule in the 'OnCalendar' setting.
This schedule has some interesting differences from cron-like patterns,
but is relatively easy to figure out from context. The one that might be
confusing is that '*-*-*' is a date-based pattern, but this must be
omitted when using 'Mon' to signal that we care about the day of the
week. Monday is used since that matches the day used for the 'weekly'
schedule used previously.

Now that the timer files are not templates, we might want to abandon the
'@' symbol in the file names. However, this would cause users with
existing schedules to get two competing schedules due to different
names. The work to remove the old schedule name is one thing that we can
avoid by keeping the '@' symbol in our unit names. Since we are locked
into this name, it makes sense that we keep the template model for the
.service files.

The rest of the change involves making sure we are writing these .timer
and .service files before initializing the schedule with 'systemctl' and
deleting the files when we are done. Some changes are also made to share
the random minute along with a single computation of the execution path
of the current Git executable.

In addition, older Git versions may have written a
'git-maintenance@.timer' template file. Be sure to remove this when
successfully enabling maintenance (or disabling maintenance).

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