Fix styling for PR merge section when no checks (#11609)

Makes styling consistent between two cases. Also removed unnecessary double border.

integrations/pull_merge_test.go

 		req := NewRequest(t, "GET", resp.Header().Get("Location"))
 		resp = session.MakeRequest(t, req, http.StatusOK)
 		htmlDoc := NewHTMLParser(t, resp.Body)
-		text := strings.TrimSpace(htmlDoc.doc.Find(".attached.header > .text.grey").Last().Text())
+		text := strings.TrimSpace(htmlDoc.doc.Find(".attached.merge-section.no-header > .text.grey").Last().Text())
 		assert.NotEmpty(t, text, "Can't find WIP text")
 
 		// remove <strong /> from lang

templates/repo/issue/view_content/pull.tmpl

 	{{- else}}red{{end}}">{{svg "octicon-git-merge" 32}}</a>
 	<div class="content">
 		{{template "repo/pulls/status" .}}
-		<div class="ui {{if not $.LatestCommitStatus}}top attached header{{else}}attached merge-section segment{{end}}">
+		<div class="ui attached merge-section segment {{if not $.LatestCommitStatus}}no-header{{end}}">
 			{{if .Issue.PullRequest.HasMerged}}
 				<div class="item text purple">
 					{{if .Issue.PullRequest.MergedCommitID}}

web_src/less/_repository.less

                     }
 
                     > .merge-section {
-                        border-top: 1px solid #d4d4d5;
                         background-color: #f7f7f7;
+
+                        .item + .item {
+                            padding-top: .5rem;
+                        }
+
+                        .divider {
+                            margin-left: -1rem;
+                            margin-right: -1rem;
+                        }
+
+                        &.no-header {
+                            #avatar-arrow;
+                        }
                     }
 
                     .markdown {