Add MERGE_OPTION and merge to mock main JH

This is fix MR for !18 (merged) and !19 (merged)

---

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2715

---

Add a new job gitlab-main-jh.

Its purpose is to merge the pre-main-jh branch into the main-jh branch.

In particular, it takes the merge parameter of ff-only： If main-jh contains new commits that are not on pre-main-jh branch, then you should synchronize pre-main-jh first, and then synchronize main-jh.

fast-forward merge can guarantee this.

And, in this MR, using mock-main-jh to verify the effect.

We need a follow up MR to stop original master-main-jh code-sync, and replace mock-main-jh to main-jh