pgsql: Improve planner's choices about when to use hashing vs sorting

Log Message:
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Improve planner's choices about when to use hashing vs sorting for DISTINCT.

The previous coding missed a bet by sometimes picking the "sorted" path
from query_planner even though hashing would be preferable. To fix, we have
to be willing to make the choice sooner. This contorts things a little bit,
but I thought of a factorization that makes it not too awful.

Modified Files:
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pgsql/src/backend/optimizer/plan:
planner.c (r1.263 -> r1.264)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/planner.c?r1=1.263&r2=1.264)