backup: Use buffered channels to collect backup status

When backing up many small files, the unbuffered channels frequently
cause the FileSaver to block when reporting progress information. Thus,
add buffers to these channels to avoid unnecessary scheduling.

As the status information is purely informational, it doesn't matter
that the status reporting shutdown is somewhat racy and could miss a few
final updates.

internal/ui/backup/progress.go

@@ -87,10 +87,13 @@ func NewProgress(printer ProgressPrinter) *Progress {
 		MinUpdatePause: time.Second / 60,
 		start:          time.Now(),
 
-		totalCh:     make(chan Counter),
-		processedCh: make(chan Counter),
+		// use buffered channels for the information used to update the status
+		// the shutdown of the `Run()` method is somewhat racy, but won't affect
+		// the final backup statistics
+		totalCh:     make(chan Counter, 100),
+		processedCh: make(chan Counter, 100),
 		errCh:       make(chan struct{}),
-		workerCh:    make(chan fileWorkerMessage),
+		workerCh:    make(chan fileWorkerMessage, 100),
 		closed:      make(chan struct{}),
 
 		summary: &Summary{},