Improve file magic handling

[jonludlam]

Before this change, the magic string was read by reading the number of bytes we're expecting to read based on our magic string. If ours happens to be a prefix of the real one (e.g. ours is 'odoc-3.1.0' and the real one is 'odoc-3.1.0-2-g12345789') then the check was succeeding and then the subsequent unmarshalling was failing.

This fixes that by writing the length too, so we always unmarshal the right length.