Clear the uniqueId of diff editor when saving

Saving diff editor transforms it to patch-file
read only editor. Clearing the uniqueId
ensures we don't use it anymore for showing
e.g. current file's modifications.

Change-Id: I2010161697bbad254ce1ebe5d0bb6a1f38ae323c
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffeditordocument.cpp

@@ -215,6 +215,7 @@ bool DiffEditorDocument::save(QString *errorString, const QString &fileName, boo
 
     setController(0);
     setDescription(QString());
+    Core::EditorManager::clearUniqueId(this);
 
     const QFileInfo fi(fileName);
     setTemporary(false);