Distinguish code flow PR creation and updates

[premun]

When we synchronize the VMR, we always create a new branch where we put the commits. For PRs that are already open, we need to distinguish this case and only add new commits on top of the branch.

Maestro will probably be able to tell us whether the PR is new or not but we probably should double check in PCS and act accordingly.

Notes: For regular Maestro PRs, we generate a branch with random GUID and we remember the branch in Maestro. We could just rely on Maestro telling us the name of the branch and we could only check the existence.