Only show "You cannot fork a repository you own" when needed (#4130)

templates/repo/header.tmpl

@@ -32,7 +32,7 @@
 				</div>
 				{{if .CanBeForked}}
 					<div class="ui compact labeled button" tabindex="0">
-						<a class="ui compact button {{if not $.CanSignedUserFork}}poping up{{end}}" {{if $.CanSignedUserFork}}href="{{AppSubUrl}}/repo/fork/{{.ID}}"{{else}} data-content="{{$.i18n.Tr "repo.fork_from_self"}}" data-position="top center" data-variation="tiny"{{end}}>
+						<a class="ui compact button {{if not $.CanSignedUserFork}}poping up{{end}}" {{if $.CanSignedUserFork}}href="{{AppSubUrl}}/repo/fork/{{.ID}}"{{else if $.IsSigned}} data-content="{{$.i18n.Tr "repo.fork_from_self"}}" data-position="top center" data-variation="tiny"{{end}}>
 							<i class="octicon octicon-repo-forked"></i>{{$.i18n.Tr "repo.fork"}}
 						</a>
 						<a class="ui basic label" href="{{.Link}}/forks">