sync: Do a git pull with --use-superproject if superproject tree already exists.

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: I7e4b1e51ca1d18b836a5fa8d139a0765262ba500