Warn when send-pack does nothing

Warn when send-pack does nothing

If you try to push into an empty repository with no ref arguments to
git push, it doesn't do anything and doesn't say anything. This adds a
warning when send-pack isn't going to push anything, so you don't
assume that it silently did what you wanted.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>