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

mercurial/configitems.py

@@ -272,6 +272,9 b" coreconfigitem('experimental', 'format.c"
 coreconfigitem('experimental', 'graphshorten',
     default=False,
 )
+coreconfigitem('experimental', 'graphstyle.parent',
+    default=dynamicdefault,
+)
 coreconfigitem('experimental', 'hook-track-tags',
     default=False,
 )