(defun sm-try-smerge ()
  (save-excursion
    (goto-char (point-min))
    (when (re-search-forward "^<<<<<<< " nil t)
      (smerge-mode 1))))

(add-hook 'find-file-hook 'sm-try-smerge t)
(add-hook 'after-revert-hook 'sm-try-smerge t)