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@gmail.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