sync: imply -c if --use-superproject option is used.

Tested the code with the following commands.

$ ./run_tests -v

Bug: [google internal] b/183232698
Bug: https://crbug.com/gerrit/13707

$ repo_dev init -u sso://android.git.corp.google.com/platform/manifest -b master --partial-clone --clone-filter=blob:limit=10M --repo-rev=main --use-superproject
$ repo_dev sync --use-superproject
$ repo_dev sync
  real	0m8.046s
  user	0m2.866s
  sys	0m2.457s

Second time repo sync took only 8 seconds and verified by printing that
urrent_branch_only is True in project.py's Sync_NetworkHalf function.

Change-Id: Ic48efb23ea427dfa36e12a5c49973d6ae776d818
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/301182


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