Fix doctest_readCurrent

It seems that OIDs are assigned in random order (dictionary iteration
order perhaps), and the test made assumptions.  Add an extra
transaction.commit() between object addition to fix OID assignment
order.

src/ZODB/tests/testConnection.py

@@ -689,9 +689,15 @@ def doctest_readCurrent():
     >>> db = ZODB.DB(store)
     >>> conn = db.open()
     >>> conn.root.a = ZODB.tests.util.P('a')
+    >>> transaction.commit()
     >>> conn.root.b = ZODB.tests.util.P('b')
     >>> transaction.commit()
 
+    >>> conn.root.a._p_oid
+    b'\x00\x00\x00\x00\x00\x00\x00\x01'
+    >>> conn.root.b._p_oid
+    b'\x00\x00\x00\x00\x00\x00\x00\x02'
+
     If we call readCurrent for an object and we modify another object,
     then checkCurrentSerialInTransaction will be called for the object
     readCurrent was called on.