tweaks: add a consistency check plus a corresponding warning

Since commit 4c6ec637 from about half a year ago, the 'action' parameter
of update_undo() is unused.  Verify that this parameter matches the type
of the item at the top of the undo stack, so that in another half year
the parameter can be safely removed.

src/text.c

@@ -1313,6 +1313,9 @@ void update_undo(undo_type action)
 	char *char_buf;
 	int char_len;
 
+	if (u->type != action)
+		statusline(ALERT, "Mismatching undo type -- please report a bug");
+
 	u->newsize = openfile->totsize;
 
 	switch (u->type) {