Mark as WIP based on MR commits only

What does this MR do?

Fixes the behaviour when an MR is automatically marked as WIP based on "fixup!" commit message. Now also non-mr commits (typically commits after a rebase) are taken in consideration.

After this fix only commits of an MR are considered as a base for WIP mark check.

Why was this MR needed?

reported bug

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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #27886 (closed)