appview: pages: introduce `ForkStatus` construct

appview/pages/pages.go

···
 	return p.executePlain("repo/fragments/repoDescription", w, params)
 }
 
+type ForkStatus int
+
+const (
+	UpToDate        ForkStatus = 0
+	FastForwardable            = 1
+	Conflict                   = 2
+	MissingBranch              = 3
+)
+
+type ForkInfo struct {
+	IsFork bool
+	Status ForkStatus
+}
+
 type RepoIndexParams struct {
 	LoggedInUser  *oauth.User
 	RepoInfo      repoinfo.RepoInfo