When a user sets up a Mirror with a restricted user that doesn't have
access to the `foo` path, we will fail to get the most recent revision
due to permissions issues.
With this change, when a prefix is provided we will get the initial
revision from the prefix rather than /foo. This allows restricted users
to setup sync.
A client-side optimization was made in #6100 to filter ascending key sorts to avoid an unnecessary re-sort since this is the order already returned by the back-end logic.
It seems to me that this really belongs on the server side since it's tied to the server implementation and should apply for any caller of the kv api (for example non-go clients).
Related, the client/v3 syncer depends on this default sorting which isn't explicit in the kv api contract. So I'm proposing the required sort parameters be included explicitly; it will take the fast path either way.
