Record each file's size on disk and print it in trees (closes #147)

The files table gets a disk_size column: the file's allocated blocks,
st_blocks × 512, from the lstat the walk already does. scan writes it
with every record it writes. trees adds it up per directory and prints
it as a column after size. On a compressed or sparse file it is less
than size, so two copies of one tree can differ; each row shows the
dupe copy's.

Judgement call: a row's disk_size is the dupe copy's, not the first's.
Unchanged files keep the disk_size from their last hash: a change in
allocation that leaves size and mtime alone is not picked up.

Model: opus-5-5

report_test.go

@@ -38,8 +38,8 @@ func awkwardPairRecs() []scanRec {
 	h, tl, c := testHash("h"), testHash("t"), testHash("c")
 
 	return []scanRec{
-		{size: 5, head: h, tail: tl, content: c, path: "/d/A/f"},
-		{size: 5, head: h, tail: tl, content: c, path: awkwardDir + "/f"},
+		{size: 5, diskSize: 4096, head: h, tail: tl, content: c, path: "/d/A/f"},
+		{size: 5, diskSize: 4096, head: h, tail: tl, content: c, path: awkwardDir + "/f"},
 	}
 }