DiffEditor: Modernize

* Use member init
* Use nullptr
* Use range-for
* omit QLatin1{String|Char} where possible

Change-Id: Ib231b747cdd9073b3d4fc6779b8e0afb2b404a31
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/diffeditor/diffeditorcontroller.h

@@ -85,9 +85,9 @@ private:
 
     Internal::DiffEditorDocument *const m_document;
 
-    bool m_isReloading;
-    int m_diffFileIndex;
-    int m_chunkIndex;
+    bool m_isReloading = false;
+    int m_diffFileIndex = -1;
+    int m_chunkIndex = -1;
 
     friend class Internal::DiffEditorDocument;
 };