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. When stdout and stderr are the same file, it clears the display
and holds its lock while writing, so no row shares a line with the
display on a shared terminal; otherwise it writes without the lock, so
a slow reader of stdout cannot stop the display. 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_test.go

@@ -284,12 +284,63 @@ func TestCloseScanDatabaseWhileReportOpen(t *testing.T) {
 
 	defer func() { _ = reportDB.Close() }()
 
-	err = loadFileRows(t.Context(), reportDB, func(scanRec) {})
+	err = loadFileRows(t.Context(), reportDB, func(int64) {},
+		func(scanRec) {})
 	if err != nil {
 		t.Fatalf("loadFileRows: %v", err)
 	}
 }
 
+// TestLoadFileRowsCountAgreesDuringScan has a scan commit records after
+// loadFileRows has counted the records and before it reads them, as
+// when trees runs during a scan: it must read exactly as many records
+// as it counted.
+func TestLoadFileRowsCountAgreesDuringScan(t *testing.T) {
+	t.Parallel()
+
+	path := testDBPath(t)
+
+	scanDB, err := openScanDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = scanDB.Close() }()
+
+	recs := smokeTreeRecs()
+
+	err = applyChanges(t.Context(), scanDB, recs[:3], nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	reportDB, err := openReportDatabase(t.Context(), path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = reportDB.Close() }()
+
+	var counted, read int64
+
+	err = loadFileRows(t.Context(), reportDB, func(n int64) {
+		counted = n
+
+		err := applyChanges(t.Context(), scanDB, recs[3:], nil, nil)
+		if err != nil {
+			t.Error(err)
+		}
+	}, func(scanRec) { read++ })
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if counted != 3 || read != counted {
+		t.Errorf("counted %d records and read %d, want 3 and 3",
+			counted, read)
+	}
+}
+
 func TestApplyChangesRoundTrip(t *testing.T) {
 	t.Parallel()