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.

manifestv2: set requires at repo creation time While it should be safe to switch to the new manifest format on an existing repo, let's keep it simple for now and make the configuration have any effect only at repo creation time. If the configuration is enabled then (at repo creation), we add an entry to requires and read that instead of the configuration from then on.

test-manifest: extract constants for binary hashes The binary hashes are used quite frequently, so let's extract constants for them so we don't have to repeat binascii.unhexlify() so often.

test-manifest: create constant for empty manifest For symmetry with manifest v2, once we add that, let's extract a constant for the empty v1 manifest.