git-repack --max-pack-size: new file statics and code restructuring

git-repack --max-pack-size: new file statics and code restructuring

Add "pack_size_limit", the limit specified by --max-pack-size,
"written_list", the list of objects written to the current pack,
and "nr_written", the number of objects in written_list.
Put "base_name" at file scope again and add forward declarations.
Move write_index_file() call from cnd_pack_objects() to
write_pack_file() since only the latter will know how
many times to call write_index_file().

Signed-off-by: Dana L. How <danahow@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>