Improve merge request versions compare logic

What does this MR do?

Changes the way how we compare between for merge request versions

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

So when I squash+rebase my commit I can get more accurate diff on what changed between versions

Screenshots (if relevant)

in discussion (below)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22368, https://gitlab.com/gitlab-org/gitlab-ce/issues/22721