fix compare button on upstream repo leading to 404 (#5877) (#5914)

templates/repo/home.tmpl

@@ -54,8 +54,8 @@
 		<div class="ui stackable secondary menu mobile--margin-between-items mobile--no-negative-margins">
 			{{if and .PullRequestCtx.Allowed .IsViewBranch}}
 				<div class="fitted item">
-					<a href="{{.BaseRepo.Link}}/compare/{{.BaseRepo.DefaultBranch | EscapePound}}...{{.Repository.Owner.Name}}:{{.BranchName | EscapePound}}">
-						<button class="ui green tiny compact button"><i class="octicon octicon-git-compare"></i></button>
+					<a href="{{.BaseRepo.Link}}/compare/{{.BaseRepo.DefaultBranch | EscapePound}}...{{ if .Repository.IsFork }}{{.Repository.Owner.Name}}{{ else }}{{ .SignedUserName }}{{ end }}:{{.BranchName | EscapePound}}">
+					<button class="ui green tiny compact button"><i class="octicon octicon-git-compare"></i></button>
 					</a>
 				</div>
 			{{end}}