index-pack: restructure pack processing into three main functions

index-pack: restructure pack processing into three main functions

The second pass in parse_pack_objects() are split into
resolve_deltas(). The final phase, fixing thin pack or just seal the
pack, is now in conclude_pack() function. Main pack processing is now
a sequence of these functions:

 - parse_pack_objects() reads through the input pack
 - resolve_deltas()     makes sure all deltas can be resolved
 - conclude_pack()      seals the output pack
 - write_idx_file()     writes companion index file
 - final()              moves the pack/index to proper place

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>