Move write_shallow_commits to fetch-pack.c

This function produces network traffic and should be in fetch-pack. It
has been in commit.c because it needs to iterate (private) graft
list. It can now do so using for_each_commit_graft().

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