Store creation timepoint in HistoryNode

src/buffer.cc

@@ -59,7 +59,9 @@ static void apply_options(OptionManager& options, const ParsedLines& parsed_line
     options.get_local_option("BOM").set(parsed_lines.bom);
 }
 
-Buffer::HistoryNode::HistoryNode(HistoryNode* parent) : parent(parent) {}
+Buffer::HistoryNode::HistoryNode(HistoryNode* parent)
+    : parent(parent), timepoint{Clock::now()}
+{}
 
 Buffer::Buffer(String name, Flags flags, StringView data,
                timespec fs_timestamp)

src/buffer.hh

@@ -1,6 +1,7 @@
 #ifndef buffer_hh_INCLUDED
 #define buffer_hh_INCLUDED
 
+#include "clock.hh"
 #include "coord.hh"
 #include "flags.hh"
 #include "safe_ptr.hh"
@@ -250,6 +251,7 @@ private:
         UndoGroup undo_group;
         Vector<std::unique_ptr<HistoryNode>, MemoryDomain::BufferMeta> childs;
         SafePtr<HistoryNode> redo_child;
+        TimePoint timepoint;
     };
 
     HistoryNode           m_history;