BUG #14479: Casting non-constant values to INTERVAL DAY TO MINUTE doesn't truncate the seconds field

The following bug has been logged on the website:

Bug reference: 14479
Logged by: Piotr Stefaniak
Email address: email(at)piotr-stefaniak(dot)me
PostgreSQL version: 9.4.10
Operating system: FreeBSD, probably irrelevant
Description:

# SELECT x.y::INTERVAL DAY TO MINUTE, (interval '02:47:15.375721')::INTERVAL
DAY TO MINUTE FROM (VALUES (interval '02:47:15.375721')) x (y);
y | interval
-----------------+----------
02:47:15.375721 | 02:47:00

I chose 9.4.10 as the version in the web form because that's what I have,
but Andrew Gierth has said that this is the behavior of versions: 9.2, 9.3,
and 9.4 but not versions 9.1, 9.5, and HEAD.