Simulate merged results pipelines for as-if-jh

Context

Closes #386585 (closed)

What does this MR do and why?

New behavior for the add-jh-files:

• If a corresponding JH branch does not exist:
  • Downloads the JH_FILES_TO_COMMIT from main-jh via the API, and stores them as job artifacts
• If a corresponding JH branch exists:
  • Try to merge the corresponding JH branch onto the main-jh branch.
    • If the merge succeeds, the job will add the resulting JH_FILES_TO_COMMIT from the merged results between main-jh and the corresponding JH branch as job artifacts
    • If the merge fails, the job will add the JH_FILES_TO_COMMIT from the corresponding JH branch only as job artifacts. The job will also fail, but will be allowed to fail.
    • If the job fails for any other reason, it will fail.

Does it work?

Without a corresponding branch in https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab

Job: https://gitlab.com/gitlab-org/gitlab/-/jobs/4869355128

With a corresponding branch in https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab

• Corresponding JH branch: 386585-have_merged_results_from_main_jh_as_if_jh-jh
  • The branch starts on commit https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab/-/commit/493157d55e2dbbd81c134cc8dc9371a298aa2493, an older commit in main-jh. The goal is to check whether the newer commits in main-jh will be included in the merged results 🤞🏼

Without conflicts

Job: https://gitlab.com/gitlab-org/gitlab/-/jobs/4869665197

Downloading the artifacts for the job, we see that the changes in https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab/-/commit/1bb337a651b78c904faa81f7c91f60d0ae65a56e are included 🎉 :

cd ~/Downloads/
unzip artifacts.zip
cd artifacts/

# We want to see whether the file `jh/doc/api/database_migrations.md` was added (introduced in https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab/-/commit/1bb337a651b78c904faa81f7c91f60d0ae65a56e#ee35beac85c01c6bec614b416aacda45dd717f5e_0_1)

 $ ls -l jh/doc/api/database_migrations.md
-rw-r--r--@ 1 gitlab  staff  1226 Aug 14 17:20 jh/doc/api/database_migrations.md

$ cat jh/doc/api/database_migrations.md
---
stage: Data Stores
group: Database
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/product/ux/technical-writing/#assignments
---

# 数据库迁移 API **(FREE SELF)**

> 引入于极狐GitLab 16.2。

该 API 用于管理极狐GitLab 开发中的数据库迁移。

所有方法都需要管理员权限。

## 将迁移标记为成功

[...]

With conflicts

• Job: https://gitlab.com/gitlab-org/gitlab/-/jobs/4873766077#L83
  • I added a commit to the corresponding branch to create a conflict with another commit in the JiHu main branch (commit added)

Additionally, the job exits with an exit code of 3, which marks it as allowed to fail 🎉

Downloading the artifacts for the job, we see that the changes in https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab/-/commit/1bb337a651b78c904faa81f7c91f60d0ae65a56e are NOT included (which is good!) 🎉 :

cd ~/Downloads/
unzip artifacts.zip

# We want to verify that https://gitlab.com/gitlab-org/gitlab-jh-mirrors/gitlab/-/commit/1bb337a651b78c904faa81f7c91f60d0ae65a56e#ee35beac85c01c6bec614b416aacda45dd717f5e_0_1 is not in the `jh` folder
$ cat jh/doc/api/database_migrations.md
This is just to create merge conflicts!

Success 🎉

With another error

• Job: https://gitlab.com/gitlab-org/gitlab/-/jobs/4873819475#L53
• I introduced an error in the as-if-jh script (see commit)

We also see that the job is just failing, not allowed to fail 🎉

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.