send-pack --keep: do not explode into loose objects on the receiving end.

send-pack --keep: do not explode into loose objects on the receiving end.

This adds "keep-pack" extension to send-pack vs receive pack protocol,
and makes the receiver invoke "index-pack --stdin --fix-thin".

With this, you can ask send-pack not to explode the result into
loose objects on the receiving end.

I've patched has_sha1_file() to re-check for added packs just
like is done in read_sha1_file() for now, but I think the static
"re-prepare" interface for packs was a mistake.  Creation of a
new pack inside a process that needs to read objects in them
back ought to be a rare event, so we are better off making the
callers (such as receive-pack that calls "index-pack --stdin
--fix-thin") explicitly call re-prepare.  That way we do not
have to penalize ordinary users of read_sha1_file() and
has_sha1_file().

We would need to fix this someday.

Signed-off-by: Junio C Hamano <junkio@cox.net>