Example:

   module M
     module N
       module C
         class D
           def C.foo
             _
           end
         end
       end
     end
   end

(ruby-add-log-current-method) currently returns "M::C.foo"

While it should return "M::N::C.foo". Patch attached.

This discovery stems from Mattias Engdegård's report (in private) about 
an ignored return value from `nreverse`.

Is this good for emacs-29?

It seems pretty safe (with decent test coverage, including the new 
test), but probably low urgency. Not a regression either.