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

The total summed `size` over every row. It now leaves out a row whose
`dupe` lies inside another row's `dupe`. Before, such a row's bytes were
counted again in each row whose `dupe` holds it. The rows are unchanged.
README defines the total to match.

Model: opus-5-5

README.md

@@ -848,7 +848,8 @@ The summary line on stderr gives:
 - the records read;
 - 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