for-each-ref: Fix sort with multiple keys

for-each-ref: Fix sort with multiple keys

The linked list describing sort options was not correctly set up in
opt_parse_sort. In the result, contrary to the documentation, only the
last of multiple --sort options to git-for-each-ref was taken into
account. This commit fixes it.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>