vim: fix count_threads variable check

It never really worked; in Ruby only 'nil' and 'false' evaluate to
false, therefore the statement '0 : true ? false' returns true, so it
doesn't matter if notmuch_folders_count_threads = 0, count_threads would
be true.

We need to check specifically if the value is 1 or 0.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>