add margin top to the top of branches (#23002)

add margin top as mentioned in #22973

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Co-authored-by: jidi <jidi@jidideMacBook-Pro.local>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/repo/branch/list.tmpl

@@ -5,7 +5,7 @@
 		{{template "base/alert" .}}
 		{{template "repo/sub_menu" .}}
 		{{if .DefaultBranchBranch}}
-			<h4 class="ui top attached header">
+			<h4 class="ui top attached header gt-mt-4">
 				{{.locale.Tr "repo.default_branch"}}
 			</h4>