bundlerepo: mark internal-only config variable

mercurial/bundlerepo.py

@@ -346,6 +346,7 b' class bundlerepository(localrepo.localre'
 def instance(ui, path, create):
     if create:
         raise util.Abort(_('cannot create new bundle repository'))
+    # internal config: bundle.mainreporoot
     parentpath = ui.config("bundle", "mainreporoot", "")
     if not parentpath:
         # try to find the correct path to the working directory repo