Fix pipeline 1491428 merge base

Depends on Upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/130256

What does this MR do and why?

Failed Pipeline: https://jihulab.com/gitlab-cn/gitlab/-/pipelines/1491428

Failed Job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/9148783

Related Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/420668

Related MRs:

• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/128177
• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/129539

In this refactor, moved the method squash_sha! from app/services/merge_requests/merge_base_service.rb to app/services/merge_requests/merge_base_service.rb,

and , the new class with no prepend_mod attached.

Screenshots or screen recordings

Merge page:

Settings	Commits

How to set up and validate locally

1. ::Feature.enable(:single_squash_merge_ff)
2. Goto project settings page, Merge Request, select "Single Squash Merge"
3. Create MRs and merge them

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.