git-bundle: avoid packing objects which are in the prerequisites

git-bundle: avoid packing objects which are in the prerequisites

When saying something like "--since=1.day.ago" or "--max-count=5",
git-bundle finds the boundary commits which are recorded as
prerequisites. However, it failed to tell pack-objects _not_ to
pack the objects which are in these.

Fix that. And add a test for that.

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