configitems: register the 'format.manifestcachesize' config

mercurial/configitems.py

@@ -116,6 +116,9 b" coreconfigitem('format', 'dotencode',"
 coreconfigitem('format', 'generaldelta',
     default=False,
 )
+coreconfigitem('format', 'manifestcachesize',
+    default=None,
+)
 coreconfigitem('hostsecurity', 'ciphers',
     default=None,
 )