/// copy a file. Return non-empty log on success, empty log on failure.
virtual std::string copy(support::FileName const &, std::string const &) = 0;
/// check in the current revision.
/// copy a file. Return non-empty log on success, empty log on failure.
virtual std::string copy(support::FileName const &, std::string const &) = 0;
/// check in the current revision.