revamp git-clone (take #2).

revamp git-clone (take #2).

This builds on top of the previous one.

 * --use-separate-remote uses .git/refs/remotes/$origin/
   directory to keep track of the upstream branches.

 * The $origin above defaults to "origin" as usual, but the
   existing "-o $origin" option can be used to override it.

I am not yet convinced if we should make "$origin" the synonym to
"refs/remotes/$origin/$name" where $name is the primary branch
name of $origin upstream, nor if so how we should decide which
upstream branch is the primary one, but that is more or less
orthogonal to what the clone does here.

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