sync: Added --use-superproject option and support for superproject.

Added "--use-superporject" option to sync.py to fetch project SHAs from
superproject. If there are any missing projects in superprojects, it
prints the missing entries and exits. If there are no missing entries,
it will use SHAs from superproject to fetch the projects from git.

Tested the code with the following commands.

$ ./run_tests tests/test_manifest_xml.py
$ ./run_tests -v tests/test_git_superproject.py
$ ./run_tests -v

Tested the sync code by copying all the repo changes into my Android
AOSP checkout and adding <superporject> tag to default.xml. With
local modification to the code to print the status,

.../WORKING_DIRECTORY$ repo sync --use-superproject
repo: executing 'git clone' url: sso://android/platform/superproject
repo: executing 'git ls-tree'
Success: []

Bug: https://crbug.com/gerrit/13709


Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: Id18665992428dd684c04b0e0b3a52f46316873a0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/293822


Reviewed-by: Mike Frysinger <vapier@google.com>