It was broken since commit 4a4d2e64
("client: optimize by not calling lastTransaction at the end of transactions").

This fixes several unit tests like
neo.tests.zodb.testBasic.BasicTests.check_tid_ordering_w_commit

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2819 71dcc9de-d417-0410-9af5-da40c76e7ee4