Fix Emacs thinking a file has changed when its contents haven't only its modification time. Useful when using git checkout

update-visited-file-modtime-if-unchanged.el

(defun update-visited-file-modtime-if-unchanged ()
  "Check the contents of all file buffers with changed file-modtime to see if they have really changed.
If not `set-visited-file-modtime' to match file."
  (interactive)
  (with-temp-buffer
    (buffer-disable-undo)
    (let (buf
          fn
          (cbuf (current-buffer))
          (buffers (buffer-list)))
      (while buffers
        (setq buf (car buffers))
        (setq buffers (cdr buffers))
      (when (and (setq fn (buffer-file-name buf)) (not (verify-visited-file-modtime buf)) (file-exists-p fn))
        (erase-buffer)
        (insert-file-contents fn)
        (when (= (buffer-size) (buffer-size buf))
          (with-current-buffer buf
            (save-restriction
              (widen)
              (when (= 0 (compare-buffer-substrings cbuf 1 (buffer-size) buf 1 (buffer-size)))
                (set-visited-file-modtime))))))))))