>> But diff-mode overrides this binding with diff-mode-shared-map
>> where the TAB key is bound to diff-hunk-next, since
>> minor-mode-overriding-map-alist takes priority over
>> minor-mode-map-alist when diff-mode does this:
>
> I see.  I knew using `minor-mode-overriding-map-alist` in `diff-mode.el`
> this way was going to bite us sooner or later.
>
> Maybe we should use a hook on `read-only-mode` to set/unset
> a `diff-mode-read-only` variable so we can add the keymap
> (conditionalized on this new `diff-mode-read-only`) to
> `minor-mode-map-alist` instead of `minor-mode-overriding-map-alist`.

So this is because `minor-mode-map-alist` is not buffer-local.
Then this requires changing `(setq buffer-read-only t)` to
`(read-only-mode 1)` in diff-related places.  Since `read-only-mode`
always activates `view-mode` when `view-read-only` is t,
it needs let-binding: (let ((view-read-only nil)) (read-only-mode 1)).
This will keep the current behavior.  Then special-handling of
`view-mode` in `diff-mode` is not needed because `view-mode`
is higher than `diff-mode-read-only` in `minor-mode-map-alist`,
where `diff-mode-read-only` is at the end to not take precedence
over `outline-minor-mode`.