Fix deduplication "single value" strategy bug.

It was possible for deduplication's single value strategy to mistakenly
believe that a very small duplicate tuple counts as one of the six large
tuples that it aims to leave behind after the page finally splits.  This
could cause slightly suboptimal space utilization with very low
cardinality indexes, though only under fairly narrow conditions.

To fix, be particular about what kind of tuple counts as a
maxpostingsize-capped tuple.  This avoids confusion in the event of a
small tuple that gets "wedged" between two large tuples, where all
tuples on the page are duplicates of the same value.

Discussion: https://postgr.es/m/CAH2-Wz=Y+sgSFc-O3LpiZX-POx2bC+okec2KafERHuzdVa7-rQ@mail.gmail.com
Backpatch: 13-, where deduplication was introduced (by commit 0d861bbb)

src/include/access/nbtree.h

@@ -739,6 +739,7 @@ typedef struct BTDedupStateData
 {
 	/* Deduplication status info for entire pass over page */
 	bool		deduplicate;	/* Still deduplicating page? */
+	int			nmaxitems;		/* Number of max-sized tuples so far */
 	Size		maxpostingsize; /* Limit on size of final tuple */
 
 	/* Metadata about base tuple of current pending posting list */