fast-import: Stream very large blobs directly to pack

fast-import: Stream very large blobs directly to pack

If a blob is larger than the configured big-file-threshold, instead
of reading it into a single buffer obtained from malloc, stream it
onto the end of the current pack file.  Streaming the larger objects
into the pack avoids the 4+ GiB memory footprint that occurs when
fast-import is processing 2+ GiB blobs.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>