enable index-pack streaming capability

enable index-pack streaming capability

A new flag, --stdin, allows for a pack to be received over a stream.
When this flag is provided, the pack content is written to either
the named pack file or directly to the object repository under the
same name as produced by git-repack.  The pack index is written as
well with the corresponding base name, unless the index name is
overriden with -o.

With this patch, git-index-pack could be used instead of
git-unpack-objects when fetching remote objects but only with
non "thin" packs for now.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>