completion: use __gitcomp_builtin in _git_ls_tree

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1629,6 +1629,13 @@ _git_ls_remote ()
 
 _git_ls_tree ()
 {
+	case "$cur" in
+	--*)
+		__gitcomp_builtin ls-tree
+		return
+		;;
+	esac
+
 	__git_complete_file
 }