Fix the problem that backup time may be negative (pingcap#405)

* backup: make sure backup time greater than 0

* backup: rename some variables

* backup: make CI happy

pkg/backup/client.go

@@ -406,13 +406,14 @@ func (bc *Client) BackupRanges(
 	allFilesCollected := make(chan struct{}, 1)
 	go func() {
 		init := time.Now()
-		start, cur := init, init
+		// nolint:ineffassign
+		lastBackupStart, currentBackupStart := init, init
 		for files := range filesCh {
-			cur, start = start, time.Now()
+			lastBackupStart, currentBackupStart = currentBackupStart, time.Now()
 			allFiles = append(allFiles, files...)
-			summary.CollectSuccessUnit("backup ranges", 1, cur.Sub(start))
+			summary.CollectSuccessUnit("backup ranges", 1, currentBackupStart.Sub(lastBackupStart))
 		}
-		log.Info("Backup Ranges", zap.Duration("take", cur.Sub(init)))
+		log.Info("Backup Ranges", zap.Duration("take", currentBackupStart.Sub(init)))
 		allFilesCollected <- struct{}{}
 	}()