Consistently truncate non-key suffix columns.

Consistently truncate non-key suffix columns.

INCLUDE indexes failed to have their non-key attributes physically
truncated away in certain rare cases.  This led to physically larger
pivot tuples that contained useless non-key attribute values.  The
impact on users should be negligible, but this is still clearly a
regression (Postgres 11 supports INCLUDE indexes, and yet was not
affected).

The bug appeared in commit dd299df8, which introduced "true" suffix
truncation of key attributes.

Discussion: https://postgr.es/m/CAH2-Wz=E8pkV9ivRSFHtv812H5ckf8s1-yhx61_WrJbKccGcrQ@mail.gmail.com
Backpatch: 12-, where "true" suffix truncation was introduced.