The revision editor builds its extensions once, on mount, and is remounted
via its `key` when they change. Since revisions.list stopped returning
content in #12822 the previous revision's data is fetched after the first
render, so the Diff extension is created on a later render — by which point
the editor has already been built without it and the key, which only
tracked `compareTo`, does not change.
The result is a revision view with neither diff highlights nor a change
count, which appears intermittent because it works whenever the previous
revision's content already happens to be in the store.
Include the diff inputs in the editor key so the editor is rebuilt when the
comparison content arrives, when the compared-against revision changes, and
when changes are toggled on or off.
Closes#12964
