transport-helper: don't feed bogus refs to export push

transport-helper: don't feed bogus refs to export push

When we want to push to a remote helper that has the
"export" capability, we collect all of the refs we want to
push and then feed them to fast-export.

However, the list of refs is actually a list of remote refs,
not local refs. The mapped local refs are included via the
peer_ref pointer. So when we add an argument to our
fast-export command line, we must be sure to use the local
peer_ref name (and if there is no local name, it is because
we are not actually sending that ref, or we may not even
have the ref at all).

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