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

scan_test.go

@@ -42,6 +42,21 @@ func writeFile(t *testing.T, dir, name string, data []byte) string {
 	return p
 }
 
+// diskSizeOf returns the bytes the file at path takes on disk, from its
+// own lstat: st_blocks counts 512-byte units.
+func diskSizeOf(t *testing.T, path string) int64 {
+	t.Helper()
+
+	var st syscall.Stat_t
+
+	err := syscall.Lstat(path, &st)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	return st.Blocks * 512
+}
+
 // pattern returns n bytes of deterministic content seeded by tag.
 func pattern(tag byte, n int) []byte {
 	data := make([]byte, n)
@@ -1416,6 +1431,46 @@ func TestScanStoresRawHashes(t *testing.T) {
 	}
 }
 
+// TestScanStoresDiskSize scans a sparse file, which takes less space on
+// disk than its length, and checks the disk_size stored for it against
+// the file's own lstat.
+func TestScanStoresDiskSize(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	db := openTestDB(t)
+	p := writeFile(t, dir, "sparse", []byte("x"))
+
+	// Truncate makes the file longer without giving it blocks for the
+	// new bytes.
+	const length = 1024 * 1024
+
+	err := os.Truncate(p, length)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	syncTree(t, db, dir)
+
+	want := diskSizeOf(t, p)
+	if want >= length {
+		t.Fatalf("the file takes %d bytes on disk, want fewer than its "+
+			"length %d", want, length)
+	}
+
+	var got int64
+
+	err = db.QueryRowContext(t.Context(),
+		"SELECT disk_size FROM files WHERE path = ?", []byte(p)).Scan(&got)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got != want {
+		t.Errorf("disk_size = %d, want %d", got, want)
+	}
+}
+
 // TestSyncScanSameSecondRewrite rewrites a file in place at the same
 // size with an mtime later in the same second as the recorded one: the
 // next scan must notice the change and re-hash the file. The operand is