Resolve "Merge requests show outdated discussions on changes tab"

⚠ Don't merge it yet. When !21062 (merged) gets merged, I'll retarget this to master. Ok to merge now.

What does this MR do?

Reimplements the once-merged (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20445/) added checks to avoid showing outdated discussions on lines that have been changed with later commits in the MR.

This is redone using the new logic from !21062 (merged) and will undergo performance testing to confirm no degradation is introduced before merging.

The added logic follows the example from backend where to show a discussion on a line, we see if all these fields match between line (current position and original) & discussion:

• baseSha
• headSha
• startSha

Good explanation of the backend procedure in slack (sorry, pvt link).

What are the relevant issue numbers?

Closes #48167 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #48167 (closed)