Move to use git_remote_push() instead of doing the steps ourselves. We
also change to accept a list of refspecs instead of just the one refspec
for the push method.
As part of this, we no longer error out if the server rejected any
updates, as this is a different concern from whether the push itself
failed or not. We do still error out if we attempt to push non-ff
updates.
f68b266e60