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

--- a/mercurial/interfaces/repository.py	Thu Feb 18 18:18:35 2021 +0100
+++ b/mercurial/interfaces/repository.py	Fri Feb 19 10:53:27 2021 +0100
@@ -1832,6 +1832,12 @@
     def savecommitmessage(text):
         pass
 
+    def register_sidedata_computer(kind, category, keys, computer):
+        pass
+
+    def register_wanted_sidedata(category):
+        pass
+
 
 class completelocalrepository(
     ilocalrepositorymain, ilocalrepositoryfilestorage