Add the ability to administratively enroll repo into using superproject.

Repo will remember a choice and an expiration time of the choice, per
user, about whether to use superproject by default. When not specified
from command line and the choice is not expired, repo would use the
user default value.

When a user default value is not present and when the system wide
enable default is provided in git's system configuration, repo would
ask the user for a confirmation which will be valid for two weeks.

git_config.py: Add support for system config. When reading system
	config, we would use --system to avoid hardcoding a path as the
	value may be different on some other distributions.

git_superproject.py: Add a new subroutine, _UseSuperproject(), which
	returns whether superproject should be used and whether it
	is from a user configuration.

	The value is determined in the following order:

	1. If the user specifies either --use-superproject or
	--no-use-superproject, then that choice is being used.

	2. If neither is specified, we would then check the saved value
	(upon repo init) and use that choice when there was a choice.

	3. We then check if there is a saved and unexpired value for
	user's choice in their ~/.gitconfig, and use the unexpired
	choice, if available.

	4. Finally, if all the above didn't give us a decision, and if
	the git system configuration is providing a rollout hint, present
	a prompt to user for their decision and save it in ~/.gitconfig.

subcmds/sync.py: Make use of the new UseSuperproject() provided by
git_superproject.py.

While there also silent stderr from git describe when determining the
version of repo.

Bug: [google internal] b/190688390
Change-Id: Iad3ee03026342ee500e5d65e2f0fa600d7637613
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/309762
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Xin Li <delphij@google.com>