pgsql: If a LIMIT is applied to a UNION ALL query, plan each UNION arm

Log Message:
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If a LIMIT is applied to a UNION ALL query, plan each UNION arm as
if the limit were directly applied to it. This does not actually
add a LIMIT plan node to the generated subqueries --- that would be
useless overhead --- but it does cause the planner to prefer fast-
start plans when the limit is small. After an idea from Phil Endecott.

Modified Files:
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pgsql/src/backend/optimizer/plan:
planner.c (r1.188 -> r1.189)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c.diff?r1=1.188&r2=1.189)
pgsql/src/backend/optimizer/prep:
prepunion.c (r1.123 -> r1.124)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c.diff?r1=1.123&r2=1.124)
pgsql/src/include/optimizer:
prep.h (r1.50 -> r1.51)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/prep.h.diff?r1=1.50&r2=1.51)