revlog: move the `deltachain` 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.

revlog: move the `_chunks` 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.

revlog: add a couple more of useful method on the inner object This will be needed for the next changeset.

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.