topic: check that repo.currenttopic is set before using it as an argument
repo.currenttopic can be an empty string, which means there's no current topic
set. This fact can be used to short-circuit some checks.
Also, this avoids passing an empty string as an argument to topic() revset.
#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 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null