Fix git_diff custom source bug with auto-session

- git_diff does not cache git_sign_count into diff_cache
when rmagatti/auto-session plugin is used to launch nvim
session with multiple open windows.
- This commit makes sure to not use diff_cache value, but
instead, uses the custom source to get git sign counts
when M.src (custome source for git_diff) is defined.

lua/lualine/components/diff/git_diff.lua

@@ -39,7 +39,7 @@ end
 ---error_code = { added = -1, modified = -1, removed = -1 }
 ---@param bufnr number|nil
 function M.get_sign_count(bufnr)
-  if bufnr then
+  if bufnr and not (type(M.src) == 'function') then
     return diff_cache[bufnr]
   end
   if M.src then