fix bug in drracket's online check syntax renaming setup

Because of a limitation in our GC tech (and because I have not
sorted out how to program around it (which should be possible))
when the "is it safe to rename this variable" check may silently
just say "yes" when it really doesn't know the correct answer.

It turns out that this was happening on every attempt to rename
after the first one (in a given online check syntax run) due to
a bug, now fixed in this commit (the "loop" didn't actually loop...)

Also, moved the creation of the thread to its own function to
make the things it closes over more syntactically apparent.
make negative blame also use "<collects>/" instead of having
the full collection path in there