builtin-push: also ask config for remote information

Now you can store your remote information in the config file like this:

[remote.upstream]
	url = me@company.com:the-project
	push = master:iceballs

[jc: fixed up to adjust a different fix for Push: lines earlier.]

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