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

mercurial/configitems.py

@@ -278,6 +278,9 b" coreconfigitem('experimental', 'graphsty"
 coreconfigitem('experimental', 'graphstyle.missing',
     default=dynamicdefault,
 )
+coreconfigitem('experimental', 'graphstyle.grandparent',
+    default=dynamicdefault,
+)
 coreconfigitem('experimental', 'hook-track-tags',
     default=False,
 )