Revert changeset c67920d78248.
It breaks 'hg merge && hg up -c REV' when files exist only in one of the
manifests, making hg throw a traceback like:
abort: repository a not found!255abort: error: Connection refused255abort: repository a not found!255abort: destination '../a' already exists255abort: repository a not found!255abort: destination 'q' already exists