branch: bump get_head_description() to the top

branch: bump get_head_description() to the top

This is a preperatory patch for 'roll show_detached HEAD into regular
ref_list'. This patch moves get_head_description() to the top so that
it can be used in print_ref_item().

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>