Improve sha1 object file writing.

Improve sha1 object file writing.

Make it much safer: we write to a temporary file, and then link that
temporary file to the final destination. This avoids all the nasty
races if several people write the same object at the same time.

It should also result in nicer on-disk layout, since it means that
objects all get created in the same subdirectory. That makes a lot
of block allocation algorithms happier, since the objects will now
be allocated from the same zone.