cat-file: teach --batch to stream blob objects

cat-file: teach --batch to stream blob objects

The regular "git cat-file -p" and "git cat-file blob" code
paths already learned to stream large blobs. Let's do the
same here.

Note that this means we look up the type and size before
making a decision of whether to load the object into memory
or stream (just like the "-p" code path does). That can lead
to extra work, but it should be dwarfed by the cost of
actually accessing the object itself. In my measurements,
there was a 1-2% slowdown when using "--batch" on a large
number of objects.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>