pgsql: Fix EXPLAIN ANALYZE bug noted by Wiebe Cazemier: although we were

Log Message:
-----------
Fix EXPLAIN ANALYZE bug noted by Wiebe Cazemier: although we were
properly advancing the CommandCounter between multiple sub-queries
generated by rules, we forgot to update the snapshot being used, so
that the successive sub-queries didn't actually see each others'
results. This is still not *exactly* like the semantics of normal
execution of the same queries, in that we don't take new transaction
snapshots and hence don't see changes from concurrently committed
commands, but I think that's OK and probably even preferable for
EXPLAIN ANALYZE.

Modified Files:
--------------
pgsql/src/backend/commands:
explain.c (r1.138 -> r1.139)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/explain.c.diff?r1=1.138&r2=1.139)
prepare.c (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/prepare.c.diff?r1=1.41&r2=1.42)