git log -g: Complain, but do not fail, when no reflogs are there

When asking "git log -g --all", clearly you want to see only those refs
that do have reflogs, but you do not want it to fail, either.

So instead of die()ing, complain about it, but move on to the other refs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>