tag: libify parse_opt_points_at()

tag: libify parse_opt_points_at()

Rename 'parse_opt_points_at()' to 'parse_opt_object_name()' and
move it from 'tag.c' to 'parse-options'. This now acts as a common
parse_opt function which accepts an objectname and stores it into
a sha1_array.

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>