BUG #15977: Inconsistent behavior in chained transactions

The following bug has been logged on the website:

Bug reference: 15977
Logged by: mtlh kdvt
Email address: emuser20140816(at)gmail(dot)com
PostgreSQL version: 12beta3
Operating system: Windows
Description:

When a ROLLBACK AND CHAIN command is executed in the implicit transaction
block, a new transaction will be started:
db=# ROLLBACK AND CHAIN;
WARNING: there is no transaction in progress
ROLLBACK
db=# ROLLBACK AND CHAIN;
ROLLBACK

However, a COMMIT AND CHAIN command won't start a new transaction:
db=# COMMIT AND CHAIN;
WARNING: there is no transaction in progress
COMMIT
db=# COMMIT AND CHAIN;
WARNING: there is no transaction in progress
COMMIT