DiffEditor: Fix parsing of mode-only change in patch

If one of the files has mode-only change, the entire patch
fails as a git patch, and is parsed as a text patch.

Because of that, the prefixes (a/, b/) are not stripped and
jumping to a change by double-clicking does not work.

Change-Id: Ib54ce4fa7aad02cb956af1f7de73d3c732ac5a89
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/diffeditor/diffutils.h

@@ -104,6 +104,7 @@ class DIFFEDITOR_EXPORT FileData {
 public:
     enum FileOperation {
         ChangeFile,
+        ChangeMode,
         NewFile,
         DeleteFile,
         CopyFile,