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/statichttprepo.py	Thu Feb 18 18:18:35 2021 +0100
+++ b/mercurial/statichttprepo.py	Fri Feb 19 10:53:27 2021 +0100
@@ -172,6 +172,7 @@
         self.names = namespaces.namespaces()
         self.filtername = None
         self._extrafilterid = None
+        self._wanted_sidedata = set()
 
         try:
             requirements = set(self.vfs.read(b'requires').splitlines())