Allow git push to delete remote ref.

Allow git push to delete remote ref.

This allows you to say

	git send-pack $URL :refs/heads/$branch

to delete the named remote branch.  The refspec $src:$dst means
replace the destination ref with the object known as $src on the
local side, so this is a natural extension to make an empty $src
mean "No object" to delete the target.

Signed-off-by: Junio C Hamano <junkio@cox.net>