Add the '--points-at' option provided by 'ref-filter'. The option lets
the user to list only branches which points at the given object.
Add documentation and tests for the same.
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>