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, from the line naming the database to
the summary line. open covers a wait on a busy database. read counts
the records first, as scan's load does, then reads them against that
count; group counts the directories, and write the groups. The display
gains a unit so that each rate names what its phase counts; every scan
phase passes files and prints as before. The tests that pinned trees'
stderr now allow the phase lines between the database line and the
summary.

Judgement call: opening the database is a phase of its own, so there are four phases, not the three the issue sketches.
Judgement call: the count and the read are separate queries, as in scan's load, so during a scan the read can end slightly past or short of its total.

Model: opus-5-5

progress.go

@@ -72,6 +72,7 @@ func phaseLabel(phases []string, phase string) string {
 // progressParams describe one phase's progress display as it starts.
 type progressParams struct {
 	label      string // the phase and its place, from phaseLabel
+	unit       string // what the phase counts, as "files"
 	total      int64  // the items the phase will complete; -1 when not known
 	estimate   bool   // total is an estimate, which the count may pass
 	waitingFor string // what the phase waits for until setTotal is called
@@ -101,6 +102,7 @@ type progress struct {
 	withBar    bool                     // bar is the library's bar, not its spinner
 	widest     int                      // the widest line bar has drawn, in columns
 	label      string
+	unit       string
 	total      int64
 	estimate   bool
 	waitingFor string
@@ -128,8 +130,9 @@ func startProgress(params progressParams, width func() int,
 	now := time.Now()
 	p := &progress{
 		width: width, stop: make(chan struct{}), done: make(chan struct{}),
-		label: params.label, total: params.total, estimate: params.estimate,
-		waitingFor: params.waitingFor, start: now, rateStart: now,
+		label: params.label, unit: params.unit, total: params.total,
+		estimate: params.estimate, waitingFor: params.waitingFor,
+		start: now, rateStart: now,
 	}
 
 	// Show the zero state at once: the first item may take minutes,
@@ -311,9 +314,9 @@ func (p *progress) finish() {
 //
 //	[3/5] hash: [1234/5000] 25%, 300 files/s, elapsed 4s, eta 13s
 //
-// without the percent when percent is false, as before the library's
-// bar, which shows its own. A phase with no total shows the items done
-// instead, or what it waits for.
+// where "files" is the phase's unit, without the percent when percent
+// is false, as before the library's bar, which shows its own. A phase
+// with no total shows the items done instead, or what it waits for.
 //
 // The phase and its place, the progress, the elapsed time and the ETA
 // are kept whole. A line wider than room leaves out the rate, or what
@@ -337,7 +340,7 @@ func (p *progress) line(room int, percent bool) (string, bool) {
 		}
 
 		head = p.label + ": " + p.itemsDone(percent) + ", "
-		extra = fmt.Sprintf("%.0f files/s", rate)
+		extra = fmt.Sprintf("%.0f %s/s", rate, p.unit)
 		tail = fmt.Sprintf("elapsed %s, eta %s", elapsed, p.eta(rate))
 	}
 
@@ -355,11 +358,11 @@ func (p *progress) line(room int, percent bool) (string, bool) {
 
 // itemsDone formats the items done, as "[1234/5000] 25%", with "~"
 // before a total that is an estimate and without the percent when
-// percent is false, or as "1234 files" when the phase has no total.
-// p.mu must be held.
+// percent is false, or as "1234 files", in the phase's unit, when the
+// phase has no total. p.mu must be held.
 func (p *progress) itemsDone(percent bool) string {
 	if p.total < 0 {
-		return fmt.Sprintf("%d files", p.count)
+		return fmt.Sprintf("%d %s", p.count, p.unit)
 	}
 
 	approx := ""