Set up remotes/origin to track all remote branches.

Set up remotes/origin to track all remote branches.

This implements the idea Daniel Barkalow came up with, to match
the remotes/origin created by clone by default to the workflow I
use myself in my guinea pig repository, to have me eat my own
dog food.

We probably would want to use either .git/refs/local/heads/*
(idea by Linus) or .git/refs/heads/origin/* instead to reduce
the local ref namespace pollution.

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