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

sidedata-exchange: add `wanted_sidedata` and `sidedata_computers` to repos Each repo will advertise the sidedata categories it requires (categories being unique and canonical), and have a set of "computers", functions to generate sidedata from `(repo, revlog, rev, previous_sidedata)`, for a given category. The set of computers can be a superset of the set of the wanted categories, but not smaller: repos are expected to be coherent in their handling of sidedata. Differential Revision: https://phab.mercurial-scm.org/D10028