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/changegroup.py	Thu Feb 18 18:18:35 2021 +0100
+++ b/mercurial/changegroup.py	Fri Feb 19 10:53:27 2021 +0100
@@ -945,6 +945,9 @@
         if bundlecaps is None:
             bundlecaps = set()
         self._bundlecaps = bundlecaps
+        if remote_sidedata is None:
+            remote_sidedata = set()
+        self._remote_sidedata = remote_sidedata
         self._isshallow = shallow
         self._fullclnodes = fullnodes