revlog: introduce an explicit tracking of what the revlog is about Since the dawn of time, people have been forced to rely to lossy introspection of the index filename to determine what the purpose and role of the revlog they encounter is. This is hacky, error prone, inflexible, abstraction-leaky, <insert-your-own-complaints-here>. In f63299ee7e4d Rapha?l introduced a new attribute to track this information: `revlog_kind`. However it is initialized in an odd place and various instances end up not having it set. In addition is only tracking some of the information we end up having to introspect in various pieces of code. So we add a new attribute that holds more data and is more strictly enforced. This work is done in collaboration with Rapha?l. The `revlog_kind` one will be removed/adapted in the next changeset. We expect to be able to clean up various existing piece of code and to simplify coming work around the newer revlog format. Differential Revision: https://phab.mercurial-scm.org/D10352