Add cancel buffer (to store cancelled operations.)

src/internal/buffer/generic.cc

@@ -72,21 +72,21 @@ void GenericBuffer::list_history(BEd &ctx) {
   }
 }
 
-HistoryBuffer *GenericBuffer::get_history(uint64_t version) {
+ReadonlyBuffer *GenericBuffer::get_history(uint64_t version) {
   uint64_t current = base_version + undo_stack.size();
   if (version < base_version)
     throw ed_error("History version has been pruned.");
   if (version < current) {
     auto &item = undo_stack[version - base_version];
-    return new HistoryBuffer(name, item.text, item.parse_state);
+    return new ReadonlyBuffer(name, item.text, item.parse_state);
   }
   if (version == current)
-    return new HistoryBuffer(name, root, parse);
+    return new ReadonlyBuffer(name, root, parse);
   uint64_t redo_offset = version - current - 1;
   if (redo_offset >= redo_stack.size())
     throw ed_error("No such history version.");
   auto &item = redo_stack[redo_stack.size() - 1 - redo_offset];
-  return new HistoryBuffer(name, item.text, item.parse_state);
+  return new ReadonlyBuffer(name, item.text, item.parse_state);
 }
 
 void GenericBuffer::snapshot(std::string action_) {