bundle2: gracefully skip 'obsmarkers' part if evolution is disabled

We would skip the part if it was fully unknown, so we should also skip it if we
know we won't be able to apply it. This will allow us to produce bundles with
obsolescence markers alongside changegroup while still being able to apply them
on any client.

mercurial/bundle2.py

@@ -1481,6 +1481,11 b' def handleobsmarker(op, inpart):'
     if op.ui.config('experimental', 'obsmarkers-exchange-debug', False):
         op.ui.write(('obsmarker-exchange: %i bytes received\n')
                     % len(markerdata))
+    # The mergemarkers call will crash if marker creation is not enabled.
+    # we want to avoid this if the part is advisory.
+    if not inpart.mandatory and op.repo.obsstore.readonly:
+        op.repo.ui.debug('ignoring obsolescence markers, feature not enabled')
+        return
     new = op.repo.obsstore.mergemarkers(tr, markerdata)
     if new:
         op.repo.ui.status(_('%i new obsolescence markers\n') % new)