multi-pack-index: store local property

multi-pack-index: store local property

A pack-file is 'local' if it is stored within the usual object
directory. If it is stored in an alternate, it is non-local.

Pack-files are stored using a 'pack_local' member in the packed_git
struct. Add a similar 'local' member to the multi_pack_index struct
and 'local' parameters to the methods that load and prepare multi-
pack-indexes.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>