Merge pull request #794 from dotcloud/780-diff-fix2

- Runtime: remove TrimLeft as it's go1.1
diff --git a/changes.go b/changes.go
index e50b887..dc1b015 100644
--- a/changes.go
+++ b/changes.go
@@ -65,7 +65,7 @@
 		file := filepath.Base(path)
 		// If there is a whiteout, then the file was removed
 		if strings.HasPrefix(file, ".wh.") {
-			originalFile := strings.TrimPrefix(file, ".wh.")
+			originalFile := file[len(".wh."):]
 			change.Path = filepath.Join(filepath.Dir(path), originalFile)
 			change.Kind = ChangeDelete
 		} else {