Add the '--points-at' option provided by 'ref-filter'. The
option lets the user to list only refs which points at the
given object.
Add documentation and tests for the same.
Based-on-patch-by: Jeff King <peff@peff.net> Mentored-by: Christian Couder <christian.couder@gmail.com> Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr> Signed-off-by: Karthik Nayak <karthik.188@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>