ReverseGraphIterator will be inlined. It doesn't make sense the iterator yields
Item = Result<..> whereas all possible errors are confined by constructor.
The working-copy revision is usually the latest commit, but it's not always
true. This patch ensures that the wc branch is emitted first so the graph node
order is less dependent on rewrites.
Custom backends may rely on networking or other unreliable implementations to support revsets, this change allows them to return errors cleanly instead of panicking.
For simplicity, only the public-facing Revset and RevsetGraph types are changed in this commit; the internal revset engine remains mostly unchanged and error-free since it cannot generally produce errors.
