DiffEditorController: Make it possible to setup task tree

Instead of reloader function. Use task tree for ShowController.
Make diff processing a part of task tree.

Change-Id: I732d0a14eaf8de7ce0d1be891fb4700b22ea24b7
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/vcsplugin.h

@@ -11,6 +11,8 @@ QT_BEGIN_NAMESPACE
 class QStandardItemModel;
 QT_END_NAMESPACE
 
+namespace Utils { class FutureSynchronizer; }
+
 namespace VcsBase {
 
 class VcsBaseSubmitEditor;
@@ -34,6 +36,8 @@ public:
 
     CommonVcsSettings &settings() const;
 
+    static Utils::FutureSynchronizer *futureSynchronizer();
+
     // Model of user nick names used for the submit
     // editor. Stored centrally here to achieve delayed
     // initialization and updating on settings change.