Add new merge type of single squash merge

Related issue:

• #2822 (closed)

• #3039 (closed)

• Dependent Upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/123508

Follow-up-bug-fix: !1799 (merged)

---

• Free
• JH-only
• SaaS and Self-managed
• Under Feature Flag: single_squash_merge_ff

What does this MR do and why?

Add new merge type for GitLab: Single squash merge.

	merge_requests_ff_only_enabled: true	merge_requests_ff_only_enabled: false
merge_requests_rebase_enabled: true	Fast-forward merge	Merge commit with semi-linear history
merge_requests_rebase_enabled: false	Single Squash merge (always squash)	Merge commit

Edge case

If the user sets the project to single squash merge,

and then turns off the feature flag,

then the configuration of this project will fall back to Fast-forward merge.

Screenshots or screen recordings

Live demo: https://drive.google.com/file/d/1cnN592RzdDJhmsNvIRYAzqCaCSoTzxoy/preview

Project settings

Create MR

Ready to merge

Merged 1

Commit List

Ready to merge and edit msg

Merged 2

Commit List

How to set up and validate locally

• checkout current branch
• turn on FF single_squash_merge_ff
• restart sidekiq and gitlab

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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