Update diff attached to discussion is Merge Request code review when new commits are pushed

Description

A code review is usually a cycle of commits and comments:

Do until the changes are approved:

• Push commits.
• Discuss in MR.

The thing is, that in the "Discussion" tab in the Merge Request, the discussion is shown with the old diff. This makes it harder to check if the new commits fixed the issue from the review, since we can't see the old comments with the new diff.

Proposal

When new commits are pushed that change the same lines as the ones commented on in a previous discussion:

• Show the new diff (in addition to, or instead of, the original diff) next to the discussion in the "Discussion" tab.
• Show the discussion next to the new changes in the "Changes" tab, do that the new diff can be examined while taking note of the previous review cycle.