Use `update_all` to only require one query per discussion to
update the notes resolved status. Some changes had to be made to
the discussion spec to accout for the fact that notes are not
individually updated now
- Preloading noteable we share the same noteable instance when more than one
discussion refers to the same noteable.
- Any other call to that object that is cached in that object will be for any
discussion.
- In those cases where merge_request_diff has all the sha stored to build a diff_refs get that
diff_refs using directly those sha instead accessing to the git repository to first get the
commits and later the sha.
