Avoid reltuples distortion in very small tables.

Consistently avoid trusting a sample of only one page at the point that
VACUUM determines a new reltuples for the target table (though only when
the table is larger than a single page).  This is follow-up work to
commit 74388a1a, which added a heuristic to prevent reltuples from
becoming distorted by successive VACUUM operations that each scan only a
single heap page (which was itself more or less a bugfix for an issue in
commit 44fa8488, which simplified VACUUM's handling of scanned pages).

The original bugfix commit did not account for certain remaining cases
that where not affected by its "2% of total relpages" heuristic.  This
happened with relations that are small enough that just one of its pages
exceeded the 2% threshold, yet still big enough for VACUUM to deem
skipping most of its pages via the visibility map worthwhile.  reltuples
could still become distorted over time with such a table, at least in
scenarios where the VACUUM command is run repeatedly and without the
table itself ever changing.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wzk7d4m3oEbEWkWQKd+gz-eD_peBvdXVk1a_KBygXadFeg@mail.gmail.com
Backpatch: 15-, where the rules for scanned pages changed.

src/backend/commands/vacuum.c

@@ -1234,31 +1234,25 @@ vac_estimate_reltuples(Relation relation,
 	if (scanned_pages >= total_pages)
 		return scanned_tuples;
 
-	/*
-	 * If scanned_pages is zero but total_pages isn't, keep the existing value
-	 * of reltuples.  (Note: we might be returning -1 in this case.)
-	 */
-	if (scanned_pages == 0)
-		return old_rel_tuples;
-
 	/*
 	 * When successive VACUUM commands scan the same few pages again and
 	 * again, without anything from the table really changing, there is a risk
 	 * that our beliefs about tuple density will gradually become distorted.
-	 * It's particularly important to avoid becoming confused in this way due
+	 * This might be caused by vacuumlazy.c implementation details, such as
-	 * to vacuumlazy.c implementation details.  For example, the tendency for
+	 * its tendency to always scan the last heap page.  Handle that here.
-	 * our caller to always scan the last heap page should not ever cause us
+	 *
-	 * to believe that every page in the table must be just like the last
+	 * If the relation is _exactly_ the same size according to the existing
-	 * page.
+	 * pg_class entry, and only a few of its pages (less than 2%) were
+	 * scanned, keep the existing value of reltuples.  Also keep the existing
+	 * value when only a subset of rel's pages <= a single page were scanned.
 	 *
-	 * We apply a heuristic to avoid these problems: if the relation is
+	 * (Note: we might be returning -1 here.)
-	 * exactly the same size as it was at the end of the last VACUUM, and only
-	 * a few of its pages (less than a quasi-arbitrary threshold of 2%) were
-	 * scanned by this VACUUM, assume that reltuples has not changed at all.
 	 */
 	if (old_rel_pages == total_pages &&
 		scanned_pages < (double) total_pages * 0.02)
 		return old_rel_tuples;
+	if (scanned_pages <= 1)
+		return old_rel_tuples;
 
 	/*
 	 * If old density is unknown, we can't do much except scale up