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

@@ -191,6 +191,18 @@ class Pipeline < ActiveRecord::Base
         .sort_by_merge_request_pipelines
     end
 
+    scope :detached_merge_pipelines, -> do
+      where(source: :merge_request).where(target_sha: nil)
+    end
+
+    scope :merge_pipelines, -> do
+      where(source: :merge_request).where.not(target_sha: nil)
+    end
+
+    scope :mergeable_merge_pipelines, -> (target_branch_sha) do
+      where(source: :merge_request, target_sha: target_branch_sha)
+    end
+
     # Returns the pipelines in descending order (= newest first), optionally
     # limited to a number of references.
     #
@@ -710,6 +722,18 @@ def default_branch?
       ref == project.default_branch
     end
 
+    def detached_merge_pipeline?
+      merge_request? && target_sha.nil?
+    end
+
+    def merge_pipeline?
+      merge_request? && target_sha.present?
+    end
+
+    def mergeable_merge_pipeline?
+      merge_pipeline? && target_sha == merge_request.target_branch_sha
+    end
+
     private
 
     def ci_yaml_from_repo