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.
# utility to setup pythonpath to point into the tested repository
export SRCDIR=`dirname $TESTDIR`
if [ -n "$PYTHONPATH" ]; then
export HGTEST_ORIG_PYTHONPATH=$PYTHONPATH
export PYTHONPATH=$SRCDIR:$PYTHONPATH
else
export PYTHONPATH=$SRCDIR
fi