pgsql: Teach datum_image_eq() about cstring datums.

Teach datum_image_eq() about cstring datums.

Bring datum_image_eq() in line with datumIsEqual() by adding support for
comparing cstring datums.

An upcoming patch that adds deduplication to the nbtree AM will use
datum_image_eq(). datum_image_eq() will need to work with all datatypes
that can be used as the storage type of a B-Tree index column, including
cstring. (cstring is used as the storage type for columns of type
"name" as a space-saving optimization.)

Discussion: https://postgr.es/m/CAH2-Wzn3Ee49Gmxb7V1VJ3-AC8fWn-Fr8pfWQebHe8rYRxt5OQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8c951687f58ad604be13e6addfd56446afb36e13

Modified Files
--------------
src/backend/utils/adt/datum.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)