mimic unpack-objects when --stdin is used with index-pack

mimic unpack-objects when --stdin is used with index-pack

It appears that git-unpack-objects writes the last part of the input
buffer to stdout after the pack has been parsed.  This looks a bit
suspicious since the last fill() might have filled the buffer up to
the 4096 byte limit and more data might still be pending on stdin,
but since this is about being a drop-in replacement for unpack-objects
let's simply duplicate the same behavior for now.

[jc: with fix-up appeared in Nico's sleep]

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