narrow: use featuresetupfuncs

This is the preferred way to register repo requirements that can be
opened because it respects the set of loaded extensions at repo open
time.

Differential Revision: https://phab.mercurial-scm.org/D2926

hgext/narrow/__init__.py

@@ -56,10 +56,12 b" configitem('experimental', 'narrowserveb"
 # Export the commands table for Mercurial to see.
 cmdtable = narrowcommands.table
 
-localrepo.localrepository._basesupported.add(changegroup.NARROW_REQUIREMENT)
+def featuresetup(ui, features):
+    features.add(changegroup.NARROW_REQUIREMENT)
 
 def uisetup(ui):
     """Wraps user-facing mercurial commands with narrow-aware versions."""
+    localrepo.featuresetupfuncs.add(featuresetup)
     narrowrevlog.setup()
     narrowbundle2.setup()
     narrowmerge.setup()