Reuse old rsync argv if rsync has to be restarted.

Reuse old rsync argv if rsync has to be restarted.

If rsync must be restarted due to an exit code of 12 or 13,
create_rsync_argv() was called even if the old rsync_argv should
be reused in this case. This (correctly) triggers the assertion

	assert(!name_of_reference_snapshot);

in create_rsync_argv(). Fix this by not calling create_rsync_argv()
if there is a reference snapshot.