Fix a regression with renaming models.
Review Request #8310 — Created July 27, 2016 and submitted
A recent change (made since the last public release) implemented fixes for renaming models. However, it resulted in a breakage in the case where a baseline contained the signature for the renamed model along with the original model. In this case, since the new name was found in the signature, the mutation was considered unnecessary and removed, but this wasn't actually correct, leading to an evolution failure. We now only remove that mutation if the new name exists and the old one does not, fixing renames for models.
Unit tests pass.
Tested against a database schema we had that failed to upgrade after
the recent fix for renames.