RemoteRepository.get_rev_id_for_revno fails on empty stacked repository

[Andrew Bennetts]

It's common to have a stacked repository that is completely empty (e.g. if pushing a stacked branch whose tip rev is already in the stacked-on repository). In this case RemoteRepository.get_rev_id_for_revno fails because the 'Repository.get_rev_id_for_revno' HPSS call returns a “nosuchrevision” error rather than a “history-incomplete” response.

Probably we should both:
 * Fix the server to reply with history-incomplete in this case (using the known-pair sent by the client)
 * Fix the client to interpret a nosuchrevision error for the revid in the known-pair as history-incomplete.

Fixing both sides means that old clients will start working in this case if the server is upgraded to a fixed version, and vice-versa.

This causes strangeness like breaking "bzr log -rN" and "bzr uncommit -r …" against stacked branches with empty repos hosted on a smart server.

A workaround is to avoid the smart calls, e.g. use SFTP or add the nosmart+ URL prefix