unpack-trees: move checkout state into check_updates

unpack-trees: move checkout state into check_updates

The checkout state was introduced via 16da134b1f9
(read-trees: refactor the unpack_trees() part, 2006-07-30). An attempt to
refactor the checkout state was done in b56aa5b268e (unpack-trees: pass
checkout state explicitly to check_updates(), 2016-09-13), but we can
go even further.

The `struct checkout state` is not used in unpack_trees apart from
initializing it, so move it into the function that makes use of it,
which is `check_updates`.

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