Use the proper buffer when recording undo

The code in InsetLabel::updateReferences code changes reference insets
in potentially several buffers. When recording undo, it is important
to use the right undo stack, otherwise crashes can ensue.

Once it is done, it is neccessary to create undo groups as needed.
This is done using UndoGroupHelper. This demonstrates a shortcoming of
UndoGroup helper: if a buffer is encountered in two seperate
occasions, two undo groups will be created for this buffer. This is
not correct.

Fixes bug #10643.

src/Undo.cpp

@@ -643,6 +643,13 @@ void Undo::recordUndoFullBuffer(CursorData const & cur)
 
 /// UndoGroupHelper class stuff
 
+/** FIXME: handle restarted groups
+ * It may happen that the buffers are visited in order buffer1,
+ * buffer2, buffer1. In this case, we want to have only one undo group
+ * in buffer1. One solution is to replace buffer_ with a set<Buffer*>,
+ * but I am not sure yet how to do it. A use case is
+ * InsetLabel::updateReferences.
+ */
 void UndoGroupHelper::resetBuffer(Buffer * buf)
 {
 	if (buf == buffer_)

src/Undo.h

@@ -135,7 +135,7 @@ private:
  */
 class UndoGroupHelper {
 public:
-	UndoGroupHelper(Buffer * buf) : buffer_(0)
+	UndoGroupHelper(Buffer * buf = 0) : buffer_(0)
 	{
 		resetBuffer(buf);
 	}

src/insets/InsetLabel.cpp

@@ -109,11 +109,13 @@ void InsetLabel::updateLabelAndRefs(docstring const & new_label,
 void InsetLabel::updateReferences(docstring const & old_label,
 		docstring const & new_label)
 {
+	UndoGroupHelper ugh;
 	Buffer::References const & refs = buffer().references(old_label);
 	Buffer::References::const_iterator it = refs.begin();
 	Buffer::References::const_iterator end = refs.end();
 	for (; it != end; ++it) {
-		buffer().undo().recordUndo(CursorData(it->second));
+		ugh.resetBuffer(it->second.buffer());
+		it->second.buffer()->undo().recordUndo(CursorData(it->second));
 		if (it->first->lyxCode() == MATH_REF_CODE) {
 			InsetMathRef * mi = it->first->asInsetMath()->asRefInset();
 			mi->changeTarget(new_label);