revlog: add a `entry_binary` method on index 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. Differential Revision: https://phab.mercurial-scm.org/D10508