[COMMITTERS] pgsql: Fix ExecReScanGatherMerge.

Fix ExecReScanGatherMerge.

Not surprisingly, since it'd never ever been tested, ExecReScanGatherMerge
didn't work.  Fix it, and add a regression test case to exercise it.

Amit Kapila

Discussion: https://postgr.es/m/CAA4eK1JkByysFJNh9M349u_nNjqETuEnY_y1VUc_kJiU0bxtaQ@mail.gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/de1ca6919ff8f50e09122a1001eee1420b047199

Modified Files
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src/backend/executor/nodeGatherMerge.c        |  1 +
src/test/regress/expected/select_parallel.out | 43 +++++++++++++++++++++++++++
src/test/regress/sql/select_parallel.sql      | 16 ++++++++++
3 files changed, 60 insertions(+)