pgsql: Still another try at fixing scanjoin_target insertion into paral

Still another try at fixing scanjoin_target insertion into parallel plans.

The previous code neglected the fact that the scanjoin_target might
carry sortgroupref labelings that we need to absorb. Instead, do
create_projection_path() unconditionally, and tweak the path's cost
estimate after the fact. (I'm now convinced that we ought to refactor
the way we account for sometimes not needing a separate projection step,
but right now is not the time for that sort of cleanup.)

Problem identified by Amit Kapila, patch by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/598aa194af2fb7f294ae4b029494a134a44be333

Modified Files
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src/backend/optimizer/plan/planner.c | 39 ++++++++++++++-------------
src/test/regress/expected/select_parallel.out | 18 ++++++++++++-
src/test/regress/sql/select_parallel.sql | 6 ++++-
3 files changed, 43 insertions(+), 20 deletions(-)