git-bundle: Make thin packs

git-bundle: Make thin packs

Thin packs are way smaller, but they rely on the receiving end to have the
base objects. However, Git's pack protocol also uses thin packs by
default. So make the packs contained in bundles thin, since bundles are
just another transport.

The patch looks a bit bigger than intended, mainly because --thin
_implies_ that pack-objects should run its own rev-list. Therefore, this
patch removes all the stuff we used to roll rev-list ourselves.

This commit also changes behaviour slightly: since we now know early
enough if a specified ref is _not_ contained in the pack, we can avoid
putting that ref into the pack. So, we don't die() here, but warn()
instead, and skip that ref.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>