topic: adjust head checking wrapping to not interfere with concurrent push
In the previous form, the new topic head was detected as affected and the push
was rejected.
We introduce a of 'test-topic-push.t' using the new race checker as there have
been many compatibility issue with this new checkers.
doc generated with sphinx. tutorial exported using sphinxedhg
http://hg.piranha.org.ua/sphinxedhg/