ref-filter: add parse_opt_merge_filter()

ref-filter: add parse_opt_merge_filter()

Add 'parse_opt_merge_filter()' to parse '--merged' and '--no-merged'
options and write macros for the same.

This is copied from 'builtin/branch.c' which will eventually be removed
when we port 'branch.c' to use ref-filter APIs.

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>