pgsql: Fix COMMIT/ROLLBACK AND CHAIN in the presence of subtransactions

Fix COMMIT/ROLLBACK AND CHAIN in the presence of subtransactions.

In older branches, COMMIT/ROLLBACK AND CHAIN failed to propagate
the current transaction's properties to the new transaction if
there was any open subtransaction (unreleased savepoint).
Instead, some previous transaction's properties would be restored.
This is because the "if (s->chain)" check in CommitTransactionCommand
examined the wrong instance of the "chain" flag and falsely
concluded that it didn't need to save transaction properties.

Our regression tests would have noticed this, except they used
identical transaction properties for multiple tests in a row,
so that the faulty behavior was not distinguishable from correct
behavior.

Commit 12d768e70 fixed the problem in v15 and later, but only rather
accidentally, because I removed the "if (s->chain)" test to avoid a
compiler warning, while not realizing that the warning was flagging a
real bug.

In v14 and before, remove the if-test and save transaction properties
unconditionally; just as in the newer branches, that's not expensive
enough to justify thinking harder.

Add the comment and extra regression test to v15 and later to
forestall any future recurrence, but there's no live bug in those
branches.

Patch by me, per bug #18118 from Liu Xiang.  Back-patch to v12 where
the AND CHAIN feature was added.

Discussion: https://postgr.es/m/18118-4b72fcbb903aace6@postgresql.org

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/555a8d9d37a27ee2a5df36f8a55dd335d42c2efa

Modified Files
--------------
src/backend/access/transam/xact.c          |  4 +--
src/test/regress/expected/transactions.out | 40 ++++++++++++++++++++++++++++++
src/test/regress/sql/transactions.sql      | 11 ++++++++
3 files changed, 53 insertions(+), 2 deletions(-)