fast-export: use an unsorted string list for extra_refs

fast-export: use an unsorted string list for extra_refs

The list extra_refs contains tags and the objects referenced by them,
so that they can be handled at the end.  When a tag references a
commit, that commit is added to the list using the same name.

Also, the function handle_tags_and_duplicates() relies on the order
the items were added to extra_refs, so clearly we do not want to
use a sorted list here.

Noticed by Miklos Vajna.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Tested-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>