On 18th of December, Tatsuo Ishii committed patch:
Add ALTER SYSTEM command to edit the server configuration file. Patch contributed by Amit Kapila. Reviewed by Hari Babu, Masao Fujii, Boszormenyi Zoltan, Andres Freund, Greg Smith and others.
On the next day, Fujii Masao committed patch:
Add tab completion for ALTER SYSTEM SET in psql.