log-tree: call load_ref_decorations() in get_name_decoration()

log-tree: call load_ref_decorations() in get_name_decoration()

Load a default set of ref name decorations at the first lookup.  This
frees direct and indirect callers from doing so.  They can still do it
if they want to use a filter or are interested in full decorations
instead of the default short ones -- the first load_ref_decorations()
call wins.

This means that the load in builtin/log.c::cmd_log_init_finish() is
respected even if --simplify-by-decoration is given, as the previously
dominating earlier load in handle_revision_opt() is gone.  So a filter
given with --decorate-refs-exclude is used for simplification in that
case, as expected.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>