'git rv sync --continue' fails if merge conflict ends up not requiring a commit

[dhermes]

git rv sync --continue fails if merge conflict ends up not requiring a commit.

Here is the relevant code:
https://github.com/GoogleCloudPlatform/git-rv/blob/8236118f961c1ff4775228c2b9058f45c65a7622/sync.py#L141

I need to do some research on this. After a merge, it seems if there are no needed changes after resolving conflicts that there should be an "empty" commit to change the parent hash, but I haven't been able to verify this behavior.

/cc @fredsa