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

main_test.go

@@ -303,8 +303,15 @@ func TestRunMissingDatabaseIsFatal(t *testing.T) {
 				t.Errorf("run(%s) = %d, want %d", name, code, exitFatal)
 			}
 
+			// trees shows its open phase, as it starts and as it ends,
+			// before the error.
+			opening := ""
+			if name == cmdTrees {
+				opening = treesOpenLine + "\n" + treesOpenLine + "\n"
+			}
+
 			want := name + ": database " + path + " (from " + databaseEnv +
-				")\n" +
+				")\n" + opening +
 				"sfdupes: " + path + ": no database (run \"sfdupes " +
 				"scan\" first, or set " + databaseEnv + ")\n"
 			if got := stderr(); got != want {
@@ -506,7 +513,7 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	// nothing here may create or read one: scan gets an operand that
 	// does not exist, which stops it before it takes the lock, and
 	// report and trees a cancelled context, which stops them before
-	// they read anything.
+	// they read anything. trees shows its open phase after the line.
 	t.Setenv(databaseEnv, "")
 
 	// Whether scan calls the database new depends on that machine too.
@@ -540,7 +547,8 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 			run: func() error {
 				return runTrees(cancelledContext(t), io.Discard)
 			},
-			want: "trees: database " + defaultDatabasePath + " (default)\n",
+			want: "trees: database " + defaultDatabasePath + " (default)\n" +
+				treesOpenLine + "\n" + treesOpenLine + "\n",
 		},
 	}
 
@@ -942,11 +950,16 @@ func TestRunTreesSucceeds(t *testing.T) {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 
-	want = "trees: database " + path + " (from " + databaseEnv + ")\n" +
-		"trees: 2 records read, 1 duplicate tree groups, 1 dupe trees, " +
-		"300 B reclaimable\n"
-	if got := stderr(); got != want {
-		t.Errorf("stderr = %q, want %q", got, want)
+	// Its phases come between the two lines; see
+	// TestRunTreesProgressNamesEachPhase.
+	first := "trees: database " + path + " (from " + databaseEnv + ")\n"
+	last := "\ntrees: 2 records read, 1 duplicate tree groups, " +
+		"1 dupe trees, 300 B reclaimable\n"
+
+	if got := stderr(); !strings.HasPrefix(got, first) ||
+		!strings.HasSuffix(got, last) {
+		t.Errorf("stderr = %q, want it to start %q and end %q",
+			got, first, last)
 	}
 
 	assertNoSidecars(t, path)