pgsql: Fix an old thinko in SS_make_initplan_from_plan, which is used

Log Message:
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Fix an old thinko in SS_make_initplan_from_plan, which is used when optimizing
a MIN or MAX aggregate call into an indexscan: the initplan is being made at
the current query nesting level and so we shouldn't increment query_level.
Though usually harmless, this mistake could lead to bogus "plan should not
reference subplan's variable" failures on complex queries.  Per bug report
from David Sanchez i Gregori.

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        subselect.c (r1.100.2.3 -> r1.100.2.4)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c.diff?r1=1.100.2.3&r2=1.100.2.4)
    pgsql/src/test/regress/expected:
        subselect.out (r1.13.2.1 -> r1.13.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/subselect.out.diff?r1=1.13.2.1&r2=1.13.2.2)
    pgsql/src/test/regress/sql:
        subselect.sql (r1.8.2.1 -> r1.8.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/subselect.sql.diff?r1=1.8.2.1&r2=1.8.2.2)