pgsql: In pg_upgrade, remove use of whichCluster, and just pass old/new

In pg_upgrade, remove use of whichCluster, and just pass old/new cluster
pointers, which simplifies the code.  This was not possible in 9.0 because
everything was in a single nested struct, but is possible now.

Per suggestion from Tom.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=6e6bee987ff4b6d650eec9f20fd477269d95e295

Modified Files
--------------
contrib/pg_upgrade/check.c           |   80 +++++++++++++++----------------
contrib/pg_upgrade/exec.c            |   10 ++--
contrib/pg_upgrade/function.c        |   17 +++----
contrib/pg_upgrade/info.c            |   86 ++++++++++++++++------------------
contrib/pg_upgrade/pg_upgrade.c      |   19 ++++----
contrib/pg_upgrade/pg_upgrade.h      |   54 +++++++--------------
contrib/pg_upgrade/relfilenode.c     |    7 +--
contrib/pg_upgrade/server.c          |   42 +++++++---------
contrib/pg_upgrade/tablespace.c      |   25 ++++------
contrib/pg_upgrade/version.c         |   10 ++--
contrib/pg_upgrade/version_old_8_3.c |   58 ++++++++++-------------
11 files changed, 181 insertions(+), 227 deletions(-)