DiffEditor: Simplify internals

Don't pass startupFile arg on every setDiffFiles.
Provide a setStartupFile() separate setter inside
DiffEditorDocument and call it directly from
DiffEditorController::setStartupFile().

Correct updateDescription() implementation.

Change-Id: Ie489431632bdabc61e2c692d09caf7b2e3990764
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/vcsbase/vcsbasediffeditorcontroller.h

@@ -46,8 +46,6 @@ protected:
     virtual void processCommandOutput(const QString &output);
 
     Utils::FilePath workingDirectory() const;
-    void setStartupFile(const QString &startupFile);
-    QString startupFile() const;
 
 private:
     friend class VcsBaseDiffEditorControllerPrivate;