retrieve 1.3 transaction from branched connection properly

retrieve 1.3 transaction from branched connection properly

Fixed regression where Alembic would fail to create a transaction properly
if the :class:`sqlalchemy.engine.Connection` were a so-called "branched"
connection, that is, one where the ``.connect()`` method had been called to
create a "sub" connection.

Change-Id: I5319838a08686ede7dc873ce5d39428b1afdf6ff
Fixes: #782