sync: Added --filter=blob:none (and no-depth) wduring git clone of superproject.

Tested the code with the following commands.

$ ./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 doing a repo sync --use-superproject twice.

.../WORKING_DIRECTORY$ repo sync --use-superproject

Bug: https://crbug.com/gerrit/13709
Bug: https://crbug.com/gerrit/13707


Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: Ieea31445ca89ba1d217e779ec7a7a2ebe81ac518