Reduce duplication in `Gitlab::Diff::PositionTracer`

What does this MR do?

This factorize similar code from Gitlab::Diff::PositionTracer

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

The code might be harder to understand now...

Why was this MR needed?

To get rid of flay offense.

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• 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?

Closes #22202 (closed)