revlog: move the `_chunk` method on the inner object...
revlog: move the `_chunk` method on the inner object
This is a necessary step before being able to move more logic around restoring
a revision content there.
For now, we do a simple patch for the perf extension logic, when the
implementation of the inner object changes, we will likely need some evolution
of the API. However this is true of many things in the perf extension. So we
will see this later.
