bug#9796: 24.0.90; Major mode selection after "save as"

[Glenn Morris]

Stefan Monnier wrote:

>> set-auto-mode is being used here though, not normal-mode.
>
> So I guess we shouldn't use set-auto-mode here.

normal-mode first switches to fundamental-mode, then changes to the
right mode, then turns font-lock off and on. Is that necessary here?
Also it does not have a "keep-mode-if-same" argument.
It seems simpler to me for set-visited-file-name just to
hack-local-variables.