Toggle reverting buffer when the file changes (Auto Revert mode).
With a prefix argument ARG, enable Auto Revert mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
Auto Revert mode is a minor mode that affects only the current
buffer. When enabled, it reverts the buffer when the file on
Use `global-auto-revert-mode' to automatically revert all buffers.
Use `auto-revert-tail-mode' if you know that the file will only grow
without being changed in the part that is already in the buffer. (fn &optional ARG)