pgsql: Fix handling of updated tuples in the MERGE statement

Fix handling of updated tuples in the MERGE statement

This branch missed the IsolationUsesXactSnapshot() check.  That led to EPQ on
repeatable read and serializable isolation levels.  This commit fixes the
issue and provides a simple isolation check for that.  Backpatch through v15
where MERGE statement was introduced.

Reported-by: Tender Wang <tndrwang@gmail.com>
Discussion: https://postgr.es/m/CAPpHfdvzZSaNYdj5ac-tYRi6MuuZnYHiUkZ3D-AoY-ny8v%2BS%2Bw%40mail.gmail.com
Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Backpatch-through: 15

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/13fab378e630f73b7bb821a211f10b66bc696525

Modified Files
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src/backend/executor/nodeModifyTable.c       |  5 +++++
src/test/isolation/expected/merge-update.out | 33 ++++++++++++++++++++++++++++
src/test/isolation/specs/merge-update.spec   |  2 ++
3 files changed, 40 insertions(+)