git-bundle: only die if pack would be empty, warn if ref is skipped

git-bundle: only die if pack would be empty, warn if ref is skipped

A use case for git-bundle expected to be quite common is this:

	$ git bundle create daily.bundle --since=10.days.ago --all

The expected outcome is _not_ to error out if only a couple of the
refs were not changed during the last 10 days.

This patch complains loudly about refs which are skipped due to the
pack not containing the corresponding objects, but dies only if
no objects would be in the pack _at all_.

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