Reviews lost when rebasing from master

[Brianna Bennett]

How to reproduce

1. make change

2. submit cr

3. someone reviews

4. someone approves

5. Somewhere along the way, another change has already been merged upstream and now cannot fast-forward

6. Run...

    git checkout master
    git pull
    git checkout $oldbranch
    git rebase master
    grr
   

Note, that grr was not run between IA and rebasing. This causes gerrit to consider this a "non-trivial" change, and wipes out the reviews/aproval.