attr: eliminate global check_all_attr array

attr: eliminate global check_all_attr array

Currently there is a reliance on 'check_all_attr' which is a global
array of 'attr_check_item' items which is used to store the value of
each attribute during the collection process.

This patch eliminates this global and instead creates an array per
'attr_check' instance which is then used in the attribute collection
process.  This brings the attribute system one step closer to being
thread-safe.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>