branch: roll show_detached HEAD into regular ref_list

branch: roll show_detached HEAD into regular ref_list

Remove show_detached() and make detached HEAD to be rolled into
regular ref_list by adding REF_DETACHED_HEAD as a kind of branch and
supporting the same in append_ref(). This eliminates the need for an
extra function and helps in easier porting of branch.c to use
ref-filter APIs.

Before show_detached() used to check if the HEAD branch satisfies the
'--contains' option, now that is taken care by append_ref().

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>