pgsql: MERGE post-commit review

MERGE post-commit review

Review comments from Andres Freund

* Consolidate code into AfterTriggerGetTransitionTable()
* Rename nodeMerge.c to execMerge.c
* Rename nodeMerge.h to execMerge.h
* Move MERGE handling in ExecInitModifyTable()
into a execMerge.c ExecInitMerge()
* Move mt_merge_subcommands flags into execMerge.h
* Rename opt_and_condition to opt_merge_when_and_condition
* Wordsmith various comments

Author: Pavan Deolasee
Reviewer: Simon Riggs

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4b2d44031f8c005f6f86364d7663858b6b5bdd14

Modified Files
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src/backend/commands/trigger.c | 192 ++++++++------
src/backend/executor/Makefile | 4 +-
src/backend/executor/README | 11 +-
src/backend/executor/{nodeMerge.c => execMerge.c} | 302 +++++++++++++++-------
src/backend/executor/execPartition.c | 9 +-
src/backend/executor/nodeModifyTable.c | 109 +-------
src/backend/optimizer/plan/setrefs.c | 16 +-
src/backend/parser/gram.y | 12 +-
src/include/executor/execMerge.h | 31 +++
src/include/executor/nodeMerge.h | 22 --
src/include/executor/nodeModifyTable.h | 1 +
11 files changed, 384 insertions(+), 325 deletions(-)