context: don't hex encode all unknown 20 char revision specs (issue4890) d3908c911d5e introduced nice hexified display of missing nodes. It did however also make missing 20 character revision specifications be shown as hex - very confusing. Users are often wrong and somehow specify revisions that don't exist. Nodes will however rarely be missing ... and they will only look like a user provided revision specification and be all ascii in 1 of 4*10**9. With this change, missing revisions will only be hexified if they really look like binary nodes. This change will thus improve the error reporting UI in the common case and only very rarely make it confusing in the opposite direction of how it was before.