remote: don't resolve HEAD in non-repository

remote: don't resolve HEAD in non-repository

The remote-config code wants to look at HEAD to mark the
current branch specially. But if we are not in a repository
(e.g., running "git archive --remote"), this makes no sense;
there is no HEAD to look at, and we have no current branch.

This doesn't really cause any bugs in practice (if you are
not in a repo, you probably don't have a .git/HEAD file),
but we should be more careful about triggering the refs code
at all in a non-repo. As we grow new ref backends, we would
not even know which backend to use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>