Persist source sha and target sha for merge pipelines

What does this MR do?

This MR let CreatePipelineService persist target_sha and source_sha. target_sha stores the SHA of the HEAD of the target branch of the merge request. source_sha stores the SHA of the HEAD of the source branch of the merge request.

This MR also introduces a few helper methods with these values.

Test in EE => https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9739

Expected usage

## How to create a detached pipeline?
CreatePipelineService.new(project, user, ref: "refs/merge-requests/iid/head", checkout_sha: merge_request.source_branch_sha)
                     .execute(:merge_request, merge_request: merge_request)

## How to create a merge pipeline?
CreatePipelineService.new(project, user, ref: "refs/merge-requests/iid/merge", checkout_sha: merge_sha, source_sha: merge_request.diff_head_sha, target_sha: merge_request.target_branch_sha)
                     .execute(:merge_request, merge_request: merge_request)

## How to evaluate if it's a detached pipeline?
pipeline.merge_request? && pipeline.target_sha.nil?

## How to evaluate if it's a merge pipeline?
pipeline.merge_request? && pipeline.target_sha.present?

## How to evaluate if the merge pipeline uses HEAD of target branch (If false, don't merge)?
pipeline.merge_request? && pipeline.target_sha == merge_request.target_branch_sha

What are the relevant issue numbers?

Related https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

.gitlab/issue_templates/Feature proposal.md

@@ -4,28 +4,28 @@
 
 ### Target audience
 
-<!--- For whom are we doing this? Include a [persona](https://design.gitlab.com/research/personas)
+<!--- For whom are we doing this? Include a [persona](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/)
 listed below, if applicable, along with its [label](https://gitlab.com/groups/gitlab-org/-/labels?utf8=%E2%9C%93&subscribed=&search=persona%3A),
 or define a specific company role, e.g. "Release Manager".
 
 Existing personas are: (copy relevant personas out of this comment, and delete any persona that does not apply)
 
-- Parker, Product Manager, https://design.gitlab.com/research/personas#persona-parker
+- Parker, Product Manager, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#parker-product-manager
 /label ~"Persona: Product Manager"
 
-- Delaney, Development Team Lead, https://design.gitlab.com/research/personas#persona-delaney
+- Delaney, Development Team Lead, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#delaney-development-team-lead
 /label ~"Persona: Development Team Lead"
 
-- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
+- Sasha, Software Developer, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#sasha-software-developer
 /label ~"Persona: Software developer"
 
-- Devon, DevOps Engineer, https://design.gitlab.com/research/personas#persona-devon
+- Devon, DevOps Engineer, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#devon-devops-engineer
 /label ~"Persona: DevOps Engineer"
 
-- Sidney, Systems Administrator, https://design.gitlab.com/research/personas#persona-sidney
+- Sidney, Systems Administrator, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#sidney-systems-administrator
 /label ~"Persona: Systems Administrator"
 
-- Sam, Security Analyst, https://design.gitlab.com/research/personas#persona-sam
+- Sam, Security Analyst, https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas#sam-security-analyst
 /label ~"Persona: Security Analyst"
 -->