topics: return early so that other if conditions don't get execute (issue5600) We were not returning after the _changetopics() call and hence the if's below it were also executed which resulted in changing of current topic. This prevent current topic to be updated even when we only affect changeset unrelated to the working copy. Not that the rebase output change as it expose a new bug (rebased changeset gain a topic)

test: extra output to understand the next changeset better

topics: move the logic to change or clear current topic into a new function It adds a new _changecurrenttopic which changes or clears the current topic.