fix(ts): preserve diff foldmethod in start_treesitter

lua/ts.lua

@@ -46,9 +46,11 @@ 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