The revlog index is already responsible for unpacking the binary entry, it would be
simpler to make it responsible for packing them. In practice the C version of
the index is already doing this internally.
We introduce a "entry_binary" method that return the binary version of an
existing revision. The method currently need to also take the revlog header to
deal with the "first revision" special case. We will introduce further refactor
in a later changeset to split that logic out.