MigrationManager: _transaction is a stack; not a queue

Correct the return value of current_transaction that treated the
_transaction field incorrectly as a queue rather than a stack like
everything else.  This completely broke the ability to commit and
rollback child transactions (correctly).  Fortunately, it could only
trigger on a "hint"-hint.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/migration.py

@@ -47,7 +47,7 @@ class MigrationManager(object):
 
     @property
     def current_transaction(self):
-        return self._transactions[0] if self._transactions else None
+        return self._transactions[-1] if self._transactions else None
 
     def compute_groups(self,
                        item,