git-repack --max-pack-size: split packs as asked by write_{object,one}()

git-repack --max-pack-size: split packs as asked by write_{object,one}()

Rewrite write_pack_file() to break to a new packfile
whenever write_object/write_one request it,  and
correct the header's object count in the previous packfile.
Change write_index_file() to write an index
for just the objects in the most recent packfile.

Signed-off-by: Dana L. How <redacted>
Signed-off-by: Junio C Hamano <redacted>