evolve: use right value for branch name when finding branch heads subbranch already formatted as "branchname:topicname", again appending it with ":topicname" doesn't not make sense. It's a little bit surprising that no tests fails though.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)