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. When a rescan finds a file with its size
and mtime unchanged but a different disk_size, it updates disk_size
alone, without reading the file. trees adds disk_size 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.

Judgement calls: each row's disk_size is the dupe copy's, not the
first's. A file whose record gets only a new disk_size counts as
unchanged.

Model: opus-5-5

report.go

@@ -21,14 +21,16 @@ const minGroupSize = 2
 
 // scanRec is one file record from the database. The signature (size,
 // head, tail, content) is the duplicate key; mtime is informational
-// only and used by scan for change detection.
+// only and used by scan for change detection. diskSize is the bytes the
+// file takes on disk, which trees adds up.
 type scanRec struct {
-	size    int64
-	mtime   time.Time
-	head    [sha256.Size]byte
-	tail    [sha256.Size]byte
-	content [sha256.Size]byte
-	path    string
+	size     int64
+	diskSize int64
+	mtime    time.Time
+	head     [sha256.Size]byte
+	tail     [sha256.Size]byte
+	content  [sha256.Size]byte
+	path     string
 }
 
 // reportPhase names one phase of report with its place among all of