Question

How to disable Emacs from checking the buffer file was changed outside the editor?

Answer 1

Emacs is really trying to help you here. Read the info page on Protection against Simultaneous Editing.

But, if you still want to avoid that message/prompt, you can redefine the function that is doing the prompting:

(defun ask-user-about-supersession-threat (fn)
  "blatantly ignore files that changed on disk"
  )
(defun ask-user-about-lock (file opponent)
  "always grab lock"
   t)

The second function there is for when two people are using Emacs to edit the same file, and would provide a similar prompt (but not the one you seemed to refer to in the question).

I’d advise against overriding the two routines, but it’s there if you want.

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On the off chance global-auto-revert-mode is on, you could disable that. Add this to your .emacs:

(global-auto-revert-mode -1)

You can tell if the mode is on by looking at the variable of the same name:

C-h v global-auto-revert-mode RET

If the value is t, then the mode is on, otherwise it is off.