revlog: add attribute on revlogs that specifies its kind
The sidedata logic needs to check whether the revlog it's working on is a
changelog, a manifest or a filelog.
Furthermore, future versions of the revlog format will most likely see a split
between the three types (i.e. they will store different information), so having
this will be useful for other future endeavors as well.
Differential Revision: https://phab.mercurial-scm.org/D10151