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

@@ -410,8 +410,8 @@ func userVersion(ctx context.Context, db *sql.DB) (int, error) {
 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.
+	// Everything goes through tx: the report connection is the only
+	// one, 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)

progress.go

@@ -297,7 +297,7 @@ func (p *progress) warnf(format string, args ...any) {
 // "sfdupes trees". It holds what is written to it and writes that to
 // out in pieces of about ioBufSize bytes, each under the display's
 // lock with the display cleared first, as warnf does for a warning: no
-// draw lands in the middle of a piece, and the next draw shows the
+// draw lands in the middle of a piece, and the next one shows the
 // display again, on the line below. Every write to it must end with a
 // newline, as a row written with one fmt.Fprintf does, so that each
 // piece ends a line. Flush writes what it holds once the last row is

progress_test.go

@@ -5,6 +5,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"testing"
 	"time"
@@ -612,14 +613,8 @@ func TestProgressWriterRowsOnOwnLines(t *testing.T) {
 		}
 	}
 
-	if len(shown) != len(want) {
-		t.Fatalf("terminal shows %d rows, want %d", len(shown), len(want))
-	}
-
-	for i := range want {
-		if shown[i] != want[i] {
-			t.Fatalf("terminal shows %q, want %q", shown[i], want[i])
-		}
+	if !slices.Equal(shown, want) {
+		t.Errorf("terminal shows rows %q, want %q", shown, want)
 	}
 }