fix bug with guard group containing multiple Guard::Rsync guards

In a guard group containing n Guard::Rsync guards, if
:run_group_on_start is true, at start time each guard not only runs
itself but also the n-l other Guard::Rsync guards via the
::Guard.guards.each loop, resulting in a total of n^2 invocations of
run_all.  This is easily fixed by skipping any Guard::Rsync guards
inside the loop.