PR#6361 continued: don't count 1 for Forward_tag, because in this case a GC can change the hash value.  Instead, avoid looping on Forward_tag cycles by another mean.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14850 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02