New variable `revert-buffer-in-progress-p'.

New variable `revert-buffer-in-progress-p'.

* lisp/files.el (after-find-file-from-revert-buffer): Remove variable.
  (after-find-file): Dont' bind it.
  (revert-buffer-in-progress-p): New variable.
  (revert-buffer): Bind it.
  Pass nil for `after-find-file-from-revert-buffer'.

* lisp/saveplace.el (save-place-find-file-hook): Use new variable
  `rever-buffer-in-progress-p', not `after-find-file-from-revert-buffer'.

* etc/NEWS: New variable `revert-buffer-in-progress-p'.