pgsql: Make Gather node projection-capable.

Make Gather node projection-capable.

The original Gather code failed to mark a Gather node as not able to
do projection, but it couldn't, even though it did call initialize its
projection info via ExecAssignProjectionInfo. There doesn't seem to
be any good reason for this node not to have projection capability,
so clean things up so that it does. Without this, plans using Gather
nodes might need to carry extra Result nodes to do projection.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8538a6307049590ddb5ba127b2ecac6308844d60

Modified Files
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src/backend/executor/nodeGather.c | 87 +++++++++++++++++++++++++++-------
src/backend/optimizer/plan/setrefs.c | 5 +-
src/include/nodes/execnodes.h | 1 +
3 files changed, 75 insertions(+), 18 deletions(-)