pgsql: Fix GiST index-only scans for opclasses with different storage t

Fix GiST index-only scans for opclasses with different storage type.

We cannot use the index's tuple descriptor directly to describe the index
tuples returned in an index-only scan. That's because the index might use
a different datatype for the values stored on disk than the type originally
indexed. As long as they were both pass-by-ref, it worked, but will not work
for pass-by-value types of different sizes. I noticed this as a crash when I
started hacking a patch to add fetch methods to btree_gist.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/55b59eda13a742f8af913734e22ecc8a21754414

Modified Files
--------------
src/backend/access/gist/gistscan.c | 34 +++++++++++++++++++++++++++-------
src/backend/access/gist/gistutil.c | 2 +-
src/include/access/gist_private.h | 2 ++
3 files changed, 30 insertions(+), 8 deletions(-)