Add index-only scan support to btree_gist.

inet, cidr, and timetz indexes still cannot support index-only scans,
because they don't store the original unmodified value in the index, but a
derived approximate value.

contrib/btree_gist/expected/varbit.out

@@ -64,3 +64,13 @@ SELECT count(*) FROM varbittmp WHERE a >   '1110100111010'::varbit;
     50
 (1 row)
 
+-- Test index-only scans
+SET enable_bitmapscan=off;
+EXPLAIN (COSTS OFF)
+SELECT a FROM bittmp WHERE a BETWEEN '1000000' and '1000001';
+                              QUERY PLAN                               
+-----------------------------------------------------------------------
+ Index Only Scan using bitidx on bittmp
+   Index Cond: ((a >= B'1000000'::"bit") AND (a <= B'1000001'::"bit"))
+(2 rows)
+