configitems: register the 'experimental.archivemetatemplate' config

mercurial/configitems.py

@@ -212,6 +212,9 b" coreconfigitem('email', 'reply-to',"
 coreconfigitem('experimental', 'allowdivergence',
     default=False,
 )
+coreconfigitem('experimental', 'archivemetatemplate',
+    default=dynamicdefault,
+)
 coreconfigitem('experimental', 'bundle-phases',
     default=False,
 )