We have an optimization that means we avoid traversing objects that
exist in the remote tracking branches of named remotes. We assume that
the server already has the LFS objects referred to by those Git objects.
However, this doesn't work for URL remotes, since there are no remote
tracking branches for them. Instead, let's make sure that we don't
traverse Git objects that are included in the old value of the ref,
which we know the server must have.
Note that this will not help the case where someone is pushing a new
branch to a URL remote; in that case, the old value is the all-zeros
value and we'll still have to traverse the entire history since we
haven't learned anything about what the server has.
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