graphmod: move the graphstyle options to experimental

I let this slip in the [ui] section during the review, as far as I understand we
don't plan to actually support customisation of the output on we are happy with
our choice.  The option are just here to help people tests various options so we
can decide which one we'll actually use.

I'm moving the config option in the experimental section to make this clearer
and avoid making them part of the public API by mistake.

mercurial/cmdutil.py

@@ -2227,8 +2227,9 b' def displaygraph(ui, repo, dag, displaye'
         'missing': graphmod.MISSINGPARENT
     }
     for name, key in edgetypes.items():
-        # experimental config: ui.graphstyle.*
-        styles[key] = ui.config('ui', 'graphstyle.%s' % name, styles[key])
+        # experimental config: experimental.graphstyle.*
+        styles[key] = ui.config('experimental', 'graphstyle.%s' % name,
+                                styles[key])
         if not styles[key]:
             styles[key] = None
     for rev, type, ctx, parents in dag:

tests/test-glog.t

@@ -2420,7 +2420,7 b' change graph edge styling'
 
   $ cd repo
   $ cat << EOF >> $HGRCPATH
-  > [ui]
+  > [experimental]
   > graphstyle.parent = |
   > graphstyle.grandparent = :
   > graphstyle.missing =