bug fix: when applying change operations in reverse, apply their change units in reverse order

src/core/Buffer.cc

@@ -387,9 +387,21 @@ void Buffer::save_current_operation()
 
 void Buffer::apply_change_operation(const ChangeOperation & change_operation, bool forward)
 {
-    for (auto change_unit : change_operation.changes)
+    if (forward)
     {
-        apply_change_unit(change_unit, forward);
+        for (auto change_unit : change_operation.changes)
+        {
+            apply_change_unit(change_unit, forward);
+        }
+    }
+    else
+    {
+        for (auto i = change_operation.changes.rbegin();
+             i != change_operation.changes.rend();
+             i++)
+        {
+            apply_change_unit(*i, forward);
+        }
     }
 }