sha1_file.c: move the core logic of find_pack_entry() into fill_pack_entry()

The new helper function implements the logic to find the offset for the
object in one pack and fill a pack_entry structure. The next patch will
restructure the loop and will call the helper from two places.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>