configitems: register the 'experimental.mmapindexthreshold' config

mercurial/configitems.py

@@ -197,6 +197,9 b" coreconfigitem('experimental', 'editortm"
 coreconfigitem('experimental', 'maxdeltachainspan',
     default=-1,
 )
+coreconfigitem('experimental', 'mmapindexthreshold',
+    default=None,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )

mercurial/localrepo.py

@@ -605,7 +605,7 b' class localrepository(object):'
         if 0 <= chainspan:
             self.svfs.options['maxdeltachainspan'] = chainspan
         mmapindexthreshold = self.ui.configbytes('experimental',
-                                                 'mmapindexthreshold', None)
+                                                 'mmapindexthreshold')
         if mmapindexthreshold is not None:
             self.svfs.options['mmapindexthreshold'] = mmapindexthreshold