We request Gitaly in a N+1 manner to build discussion diffs. Once the diffs are from different revisions, it's hard to make a single request to the service in order to build the whole response.
With this change we solve this problem and simplify a lot fetching this piece of info.
It appears that some legacy diff notes on GitLab.com have invalid line codes,
which means truncated_diff_lines won't work. This is just a band-aid solution,
as neither the diff nor the discussion will display correctly for these notes,
but they won't cause a 500 error.
