trees: count a nested dupe tree once in the reclaimable total (closes #168) (#188)

README.md

@@ -885,7 +885,8 @@ The summary line on stderr gives:
   operands;
 - the duplicate-tree groups;
 - the dupe trees, which is the number of rows;
-- the reclaimable bytes, the sum of `size` over all rows.
+- the reclaimable bytes, the sum of `size` over all rows, leaving out a row
+  whose `dupe` lies inside another row's `dupe`.
 
 ```
 trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimable