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

app/models/ci/pipeline.rb

@@ -622,6 +622,8 @@ def predefined_variables
         variables.append(key: 'CI_COMMIT_DESCRIPTION', value: git_commit_description.to_s)
 
         if merge_request? && merge_request
+          variables.append(key: 'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA', value: source_sha.to_s)
+          variables.append(key: 'CI_MERGE_REQUEST_TARGET_BRANCH_SHA', value: target_sha.to_s)
           variables.concat(merge_request.predefined_variables)
         end
       end