Call ask-user-about-supersession-threat only if the buffer is actually obsolete.

ask-user-about-supersession-threat-if-necessary.el

(defadvice ask-user-about-supersession-threat (around ask-user-about-supersession-threat-if-necessary)
  "Call ask-user-about-supersession-threat only if the buffer is actually obsolete."
  (if (or (buffer-modified-p)
          (verify-visited-file-modtime)
          (< (* 8 1024 1024) (buffer-size))
          (/= 0 (call-process-region 1 (+ 1 (buffer-size)) "diff" nil nil nil "-q" (buffer-file-name) "-")))
      ad-do-it
    (clear-visited-file-modtime)
    (not-modified)))
(ad-activate 'ask-user-about-supersession-threat)