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/assets/javascripts/frequent_items/components/frequent_items_list_item.vue

@@ -82,8 +82,14 @@ export default {
   <li class="frequent-items-list-item-container">
     <a :href="webUrl" class="clearfix">
       <div class="frequent-items-item-avatar-container">
-        <img v-if="hasAvatar" :src="avatarUrl" class="avatar s32" />
-        <identicon v-else :entity-id="itemId" :entity-name="itemName" size-class="s32" />
+        <img v-if="hasAvatar" :src="avatarUrl" class="avatar rect-avatar s32" />
+        <identicon
+          v-else
+          :entity-id="itemId"
+          :entity-name="itemName"
+          size-class="s32"
+          class="rect-avatar"
+        />
       </div>
       <div class="frequent-items-item-metadata-container">
         <div :title="itemName" class="frequent-items-item-title" v-html="highlightedItemName"></div>