GitClient: fix diff when committing

When a commit has several files, opening diff for the first file
works, but trying to open another diff without closing the previous
one doesn't refresh the diff editor.

Change-Id: Ia973cb70f2355a89b70787dd97042ef9f751d9d6
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Nikita Baryshnikov <nib952051@gmail.com>

src/plugins/diffeditor/diffeditorcontroller.cpp

@@ -48,7 +48,6 @@ DiffEditorController::DiffEditorController(Core::IDocument *document) :
     m_chunkIndex(-1)
 {
     QTC_ASSERT(m_document, return);
-    QTC_CHECK(!m_document->controller());
     m_document->setController(this);
 }