Merge branch '22202-reduce-duplication-in-gitlab-diff-position_tracer' into 'master'

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
  - [x] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #22202

See merge request !6365

.flayignore

 *.erb
 lib/gitlab/sanitizers/svg/whitelist.rb
+lib/gitlab/diff/position_tracer.rb