feat(undotree): set 'filetype' #36091

runtime/pack/dist/opt/nvim.undotree/lua/undotree.lua

@@ -401,6 +401,8 @@ function M.open(opts)
       end
     end,
   })
+
+  vim.bo[b].filetype = 'nvim-undotree'
 end
 
 return M