The core code is factored out and take 'struct index_state *' instead so
that we can reuse it to add objects from index files other than .git/index
in the next patch.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>