topic review irrevocably "captures" changes

• changes are pushed to a new topic
• accidentally, some of these changes are re-pushed to a new topic, together with some new changes
• it becomes impossible to "free" changes again to re-integrate them into the old topic

this is a bit sketchy because i'm giving only a 2nd hand account of http://codereview.qt-project.org/#/t/15/ 's history. you need to play around a bit to figure out what exactly goes wrong. at some point, you'll also run into QTQAINFRA-382 during the experiments (that's what happened with t15 in the end).