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.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null