submodule: store new submodule commits oid_array in a struct

submodule: store new submodule commits oid_array in a struct

This commit prepares for a future commit that will teach `git fetch
--recurse-submodules` how to fetch submodules that are present in
<gitdir>/modules, but are not populated. To do this, we need to store
more information about the changed submodule so that we can read the
submodule configuration from the superproject commit instead of the
filesystem.

Refactor the changed submodules string_list.util to hold a struct
instead of an oid_array. This struct only holds the new_commits
oid_array for now; more information will be added later.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>