fix(ts): preserve diff foldmethod in start_treesitter

lua/ts.lua

@@ -46,10 +46,12 @@ local function start_treesitter(buf)
         return
     end
     for _, win in ipairs(vim.fn.win_findbuf(buf)) do
+        if not vim.wo[win].diff then
             vim.wo[win].foldmethod = "expr"
             vim.wo[win].foldexpr = "v:lua.vim.treesitter.foldexpr()"
         end
     end
+end
 
 ---@param lang string
 local function activate_open_buffers(lang)