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.

src/include/access/gist_private.h

@@ -78,6 +78,8 @@ typedef struct GISTSTATE
 	MemoryContext tempCxt;		/* short-term context for calling functions */
 
 	TupleDesc	tupdesc;		/* index's tuple descriptor */
+	TupleDesc	fetchTupdesc;	/* tuple descriptor for tuples returned in an
+								 * index-only scan */
 
 	FmgrInfo	consistentFn[INDEX_MAX_KEYS];
 	FmgrInfo	unionFn[INDEX_MAX_KEYS];