git_remote_helpers: push all refs during a non-local export

When a remote helper exports to a non-local git repo, the
steps are roughly:

  1. fast-export into a local staging area; the set of
     interesting refs is defined by what is in the fast-export
     stream

  2. git push from the staging area to the non-local repo

In the second step, we should explicitly push all refs, not
just matching ones. This will let us push refs that do not
yet exist in the remote repo.

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>