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/concerns/sha_attribute.rb

@@ -16,6 +16,11 @@ def sha_attribute(name)
     # the column is the correct type.  In production it should behave like any other attribute.
     # See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5502 for more discussion
     def validate_binary_column_exists!(name)
+      unless database_exists?
+        warn "WARNING: sha_attribute #{name.inspect} is invalid since the database doesn't exist - you may need to create database at first"
+        return
+      end
+
       unless table_exists?
         warn "WARNING: sha_attribute #{name.inspect} is invalid since the table doesn't exist - you may need to run database migrations"
         return
@@ -35,5 +40,13 @@ def validate_binary_column_exists!(name)
       Gitlab::AppLogger.error "ShaAttribute initialization: #{error.message}"
       raise
     end
+
+    def database_exists?
+      ActiveRecord::Base.connection
+    rescue ActiveRecord::NoDatabaseError
+      false
+    else
+      true
+    end
   end
 end