Allows extraction of information about remotes.

We want to know the list of declared remotes, the local branches they
hold, and which remotes holds a given branch.  All this regardless of
where the information is stored.

If there are any git-1.5 remotes declared in .git/config, we suppose
you know what you're doing and they will take precedence on any
pre-1.5 remotes.

This does not use git-remote for now, since it is 1.5 material not
released yet, does not support legacy branches/ remotes, and does not
allow yet to query all of the information we need.

Signed-off-by: Yann Dirson <ydirson@altern.org>