topic-revset: strictly read string
Making a clear distinction between string and symbol passed to topic is
important when it comes to empty topic. Filtering an empty topic name is
harmless. Searching a non existent revision is not.
#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