pgsql: Un-break EXPLAIN for Append plans.

Log Message:
-----------
Un-break EXPLAIN for Append plans.  I messed this up a few days ago while
adding the ModifyTable node type --- I had been thinking ModifyTable should
replace Append as a special case in push_plan(), but actually both of them
have to be special-cased.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.312 -> r1.313)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c?r1=1.312&r2=1.313)