In the manifest line, flags are put directly after the hash, so the
parser has been guessing the presence of flags based on the length of
the hash. Replace this assumption by an enumeration of the valid flags
and removing them from the hash first as they are distinct input values.
Consistently handle the expected 256bit length of the SHA1-replacement
in the pure Python parser. Check that setting flags will use one of the
Extend write logic in the C version to handle 256bit hashes as well.
Verify that hashes always have exactly the expected length. Since
1070df141718 we should no longer depend on the old extra-byte hack.