configitems: register the 'format.chunkcachesize' config

mercurial/configitems.py

@@ -107,6 +107,9 b" coreconfigitem('devel', 'strip-obsmarker"
 coreconfigitem('format', 'aggressivemergedeltas',
     default=False,
 )
+coreconfigitem('format', 'chunkcachesize',
+    default=None,
+)
 coreconfigitem('hostsecurity', 'ciphers',
     default=None,
 )