Replace restic.Progress with new progress.Counter

This fixes two race conditions while cleaning up the code.

internal/ui/progress/counter_test.go

@@ -0,0 +1,55 @@
+package progress_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/restic/restic/internal/test"
+	"github.com/restic/restic/internal/ui/progress"
+)
+
+func TestCounter(t *testing.T) {
+	const N = 100
+
+	var (
+		finalSeen  = false
+		increasing = true
+		last       uint64
+		ncalls     int
+	)
+
+	report := func(value uint64, d time.Duration, final bool) {
+		finalSeen = true
+		if value < last {
+			increasing = false
+		}
+		last = value
+		ncalls++
+	}
+	c := progress.New(10*time.Millisecond, report)
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		for i := 0; i < N; i++ {
+			time.Sleep(time.Millisecond)
+			c.Add(1)
+		}
+	}()
+
+	<-done
+	c.Done()
+
+	test.Assert(t, finalSeen, "final call did not happen")
+	test.Assert(t, increasing, "values not increasing")
+	test.Equals(t, uint64(N), last)
+
+	t.Log("number of calls:", ncalls)
+}
+
+func TestCounterNil(t *testing.T) {
+	// Shouldn't panic.
+	var c *progress.Counter = nil
+	c.Add(1)
+	c.Done()
+}