BUG #16939: Plural interval for negative singular

The following bug has been logged on the website:

Bug reference:      16939
Logged by:          Max Neverov
Email address:      neverov.max@gmail.com
PostgreSQL version: 13.2
Operating system:   alpine
Description:        

Execute the query:
postgres=# set intervalstyle='postgres';
SET
postgres=#  select interval '-1 year -1 day';
     interval
------------------
 -1 years -1 days
(1 row)

Expected output:
-1 year -1 day

The code
(https://github.com/postgres/postgres/blob/master/src/backend/utils/adt/datetime.c#L4193)
pluralizes a time unit if the value is not 1, should check both -1 and 1.