[PATCH] helpful error message when send-pack finds no refs in common.

Hi all,

Please cc me, I'm not subscribed.

git-push fails when pushing into an empty repository if you don't specify
a branch:

	git --bare init some-repo

	git-push some-repo

To get this to work, you need to do

	git-push some-repo master

I attached a patch so that the error message suggests trying the latter.

Cheers,
Andrew