Discussions (grouped comments) for diff notes in merge request shows too much code

Created by: kennytm

Steps to reproduce:

1. Create a commit that changes a large part of a file (so that the diff is long).
2. Add a note to the end of the diff.
3. File a merge request for that commit.
4. Check the MR, we see that all code from the first line of the diff to that note is included:

Showing ±3 lines around the note should be enough.