trees: show its phases as N of M, with progress, elapsed time and ETA (closes #113)

trees now runs in four phases, open, read, group and write, each under
the progress display scan uses, until the summary line. open covers a
wait on a busy database. read counts the records and reads them in one
read-only transaction, so the two agree during a scan; group counts the
directories, and write the groups. The rows reach stdout through a
writer that clears the display and holds its lock while writing, so no
row shares a line with the display on a shared terminal. Each rate
names what its phase counts.

Judgement call: opening the database is a phase of its own, so there are four phases, not the three the issue sketches.

Model: opus-5-5

db.go

@@ -403,10 +403,32 @@ func userVersion(ctx context.Context, db *sql.DB) (int, error) {
 	return v, nil
 }
 
-// loadFileRows streams every record to fn in path order: byte order,
-// which is the order of the primary key, so SQLite does not sort.
-func loadFileRows(ctx context.Context, db *sql.DB, fn func(r scanRec)) error {
-	rows, err := db.QueryContext(ctx,
+// loadFileRows gives total the number of records, then streams every
+// record to fn in path order: byte order, which is the order of the
+// primary key, so SQLite does not sort. The count and the records are
+// read in one transaction, so they agree while a scan is committing.
+func loadFileRows(ctx context.Context, db *sql.DB, total func(n int64),
+	fn func(r scanRec),
+) error {
+	// Everything goes through tx: db has one connection, so a query on
+	// db would wait for tx forever.
+	tx, err := db.BeginTx(ctx, &sql.TxOptions{ReadOnly: true})
+	if err != nil {
+		return fmt.Errorf("read records: %w", err)
+	}
+
+	defer func() { _ = tx.Rollback() }()
+
+	var n int64
+
+	err = tx.QueryRowContext(ctx, "SELECT COUNT(*) FROM files").Scan(&n)
+	if err != nil {
+		return fmt.Errorf("count records: %w", err)
+	}
+
+	total(n)
+
+	rows, err := tx.QueryContext(ctx,
 		"SELECT path, size, mtime, mtime_nsec, head, tail, content "+
 			"FROM files ORDER BY path")
 	if err != nil {