BUG #18118: bug report for COMMIT AND CHAIN feature

The following bug has been logged on the website:

Bug reference:      18118
Logged by:          Liu Xiang
Email address:      evan.lx@alibaba-inc.com
PostgreSQL version: 14.9
Operating system:   centos7
Description:        

COMMIT(280a408b48d5ee42969f981bceb9e9426c3a344c) provides a way to support
chained commits. During recent development, I found that it left a flaw.

The defect can be reproduced as follows:

START TRANSACTION ISOLATION LEVEL SERIALIZABLE; COMMIT;
START TRANSACTION ISOLATION LEVEL REPEATABLE READ\; savepoint s\; COMMIT AND
CHAIN;
SHOW transaction_isolation; -- transaction is active at this point
COMMIT;

will get:
postgres=*# SHOW transaction_isolation; -- transaction is active at this
point
  transaction_isolation
-----------------------
  serializable
(1 row)

The reason for this bug is that in the case of
CommitTransactionCommand->TBLOCK_SUBCOMMIT, if there is a sub-transaction,
only RestoreTransactionCharacteristics will be performed instead of
SaveTransactionCharacteristics; this results in a global variable
save_XactIsoLevel remaining from the previous transaction being restored in
the transaction.

Thank you to the community for your continued help