Purge history on buffer reload when NoUndo flag is on

We were preserving the history in that case, so on fifo buffers
(that set the NoUndo flag until the fifo is closed), we still had
the history from the "previous life" of the buffer, leading crashes
when trying to apply it.

Fixes #1518

src/buffer.cc

@@ -242,10 +242,13 @@ void Buffer::reload(StringView data, timespec fs_timestamp)
 
     if (not record_undo)
     {
+        // Erase history about to be invalidated history
+        m_history_cursor = &m_history;
+        m_last_save_history_cursor = &m_history;
+        m_history = HistoryNode{m_next_history_id++, nullptr};
+
         m_changes.push_back({ Change::Erase, {0,0}, line_count() });
-
         static_cast<BufferLines&>(m_lines) = std::move(parsed_lines.lines);
-
         m_changes.push_back({ Change::Insert, {0,0}, line_count() });
     }
     else
@@ -340,7 +343,7 @@ bool Buffer::redo(size_t count) noexcept
 
     while (count-- != 0 and m_history_cursor->redo_child)
     {
-        m_history_cursor = m_history_cursor->redo_child.get();
+        m_history_cursor = m_history_cursor->redo_child;
 
         for (const Modification& modification : m_history_cursor->undo_group)
             apply_modification(modification);

src/buffer.hh

@@ -259,7 +259,7 @@ private:
         UndoGroup undo_group;
         Vector<std::unique_ptr<HistoryNode>, MemoryDomain::BufferMeta> childs;
         SafePtr<HistoryNode> parent;
-        SafePtr<HistoryNode> redo_child;
+        HistoryNode* redo_child = nullptr; // not a SafePtr to avoid lifetime issues between this and childs
         size_t id;
         TimePoint timepoint;
     };

test/regression/1518-wrong-undo-handling-with-fifo-buffers/cmd

@@ -0,0 +1 @@
+<a-O>ifoo<c-u><esc>:edit -fifo /dev/null <c-r>%<ret>u