Make maximal use of the remote refs

Make maximal use of the remote refs

When git-fetch-pack gets the remote refs, it does not need to filter them
right away, but it can see which refs are common (taking advantage of the
patch which makes git-fetch-pack not use git-rev-list).

This means that we ask get_remote_heads() to return all remote refs,
including the funny refs, and filtering them with a separate function later.

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