sync: use superproject if manifest's config has superproject enabled.

If --use-superproject is passed as argument to "repo init", then
--use-superproject need not be specified during "repo sync".

Tested the code with the following commands.

$ time repo_dev sync -c -j32
...
WARNING: --use-superproject is experimental and not for general use

Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707
Change-Id: Ibb33f3038a2515f74a6c4f7cb785d354b26ee680
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298102


Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Ian Kasprzak <iankaz@google.com>