fix(Dgraph): Sort manifests by BackupNum in file handler. (#6263)

This was being done in the S3 handler but not in the file handler.
By default the paths are sorted by the filename but sorting by backup
number is more robust.

worker/file_handler.go

@@ -139,6 +139,11 @@ func (h *fileHandler) GetManifests(uri *url.URL, backupId string) ([]*Manifest,
 		return nil, err
 	}
 
+	// Sort manifests in the ascending order of their BackupNum so that the first
+	// manifest corresponds to the first full backup and so on.
+	sort.Slice(manifests, func(i, j int) bool {
+		return manifests[i].BackupNum < manifests[j].BackupNum
+	})
 	return manifests, nil
 }