configitems: register the 'experimental.graphstyle.missing' config

mercurial/configitems.py

@@ -275,6 +275,9 b" coreconfigitem('experimental', 'graphsho"
 coreconfigitem('experimental', 'graphstyle.parent',
     default=dynamicdefault,
 )
+coreconfigitem('experimental', 'graphstyle.missing',
+    default=dynamicdefault,
+)
 coreconfigitem('experimental', 'hook-track-tags',
     default=False,
 )