pg_upgrade: Avoid shadowing global var in function

pg_upgrade: Avoid shadowing global var in function

The new_cluster parameter in check_for_new_tablespace_dir was
shadowing the globally defined new_cluster variable, causing
compiler warnings when running with -Wshadow. The function is
only applicable to the new cluster, so remove the parameter
rather than rename to match check_new_cluster_is_empty which
also only applies to the new cluster.

Author: Peter Smith <peter.b.smith@fujitsu.com>
Discussion: https://postgr.es/m/CAHut+PvS_PHLntWy1yTgXv0O1tWm4iVcKBQFzpoQRDsm2Ce_Fg@mail.gmail.com