configitems: register the 'experimental.maxdeltachainspan' config

mercurial/configitems.py

@@ -194,6 +194,9 b" coreconfigitem('experimental', 'crecordt"
 coreconfigitem('experimental', 'editortmpinhg',
     default=False,
 )
+coreconfigitem('experimental', 'maxdeltachainspan',
+    default=-1,
+)
 coreconfigitem('experimental', 'nonnormalparanoidcheck',
     default=False,
 )

mercurial/localrepo.py

@@ -601,7 +601,7 b' class localrepository(object):'
                                                    'aggressivemergedeltas')
         self.svfs.options['aggressivemergedeltas'] = aggressivemergedeltas
         self.svfs.options['lazydeltabase'] = not scmutil.gddeltaconfig(self.ui)
-        chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan', -1)
+        chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan')
         if 0 <= chainspan:
             self.svfs.options['maxdeltachainspan'] = chainspan
         mmapindexthreshold = self.ui.configbytes('experimental',