manifestv2: add support for reading new manifest format The new manifest format is designed to be smaller, in particular to produce smaller deltas. It stores hashes in binary and puts the hash on a new line (for smaller deltas). It also uses stem compression to save space for long paths. The format has room for metadata, but that's there only for future-proofing. The parser thus accepts any metadata and throws it away. For more information, see http://mercurial.selenic.com/wiki/ManifestV2Plan. The current manifest format doesn't allow an empty filename, so we use an empty filename on the first line to tell a manifest of the new format from the old. Since we still never write manifests in the new format, the added code is unused, but it is tested by test-manifest.py.