unpack-trees: also allow get_progress() to work on a different index

unpack-trees: also allow get_progress() to work on a different index

commit b0a5a12a60 ("unpack-trees: allow check_updates() to work on a
different index", 2020-03-27) allowed check_updates() to work on a
different index, but it called get_progress() which was hardcoded to
work on o->result much like check_updates() had been.  Update it to also
accept an index parameter and have check_updates() pass that parameter
along so that both are working on the same index.

Noticed-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>