maintenance: git maintenance run learned --scheduler=<scheduler>

Depending on the system, different schedulers can be used to schedule
the hourly, daily and weekly executions of `git maintenance run`:
* `launchctl` for MacOS,
* `schtasks` for Windows and
* `crontab` for everything else.

`git maintenance run` now has an option to let the end-user explicitly
choose which scheduler he wants to use:
`--scheduler=auto|crontab|launchctl|schtasks`.

When `git maintenance start --scheduler=XXX` is run, it not only
registers `git maintenance run` tasks in the scheduler XXX, it also
removes the `git maintenance run` tasks from all the other schedulers to
ensure we cannot have two schedulers launching concurrent identical
tasks.

The default value is `auto` which chooses a suitable scheduler for the
system.

`git maintenance stop` doesn't have any `--scheduler` parameter because
this command will try to remove the `git maintenance run` tasks from all
the available schedulers.

Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: L3n41c

Documentation/git-maintenance.txt

@@ -179,6 +179,15 @@ OPTIONS
 	`maintenance.<task>.enabled` configured as `true` are considered.
 	See the 'TASKS' section for the list of accepted `<task>` values.
 
+--scheduler=auto|crontab|launchctl|schtasks::
+	When combined with the `start` subcommand, specify the scheduler
+	for running the hourly, daily and weekly executions of
+	`git maintenance run`.
+	Possible values for `<scheduler>` are `auto`, `crontab` (POSIX),
+	`launchctl` (macOS), and `schtasks` (Windows).
+	When `auto` is specified, the appropriate platform-specific
+	scheduler is used. Default is `auto`.
+
 
 TROUBLESHOOTING
 ---------------