We create/update the `DiffNotePosition` of a discussion (via the
first note ID), whenever a MR has been updated and we check for
mergeability of a MR. This means that when a merge request changes
the `DiffNotePosition` can possibly change.

Previously, when that happens, the cache will be stale and the
`positions` property we return in `discussions.json` will be outdated
and won't match anything on the new HEAD diff so discussions that
supposed to be still displayed won't be displayed.

This fix adds the SHA of `DiffNotePosition#position` records of
each `DiffDiscussion` to the cache key. This way, if they change,
the cache for the corresponding discussion will be invalidated.