On the 'moves-tracking-2' branch: add a simple constructor.

* subversion/include/private/svn_editor3.h,
  subversion/libsvn_delta/editor3.c
  (svn_editor3_node_content_create_ref): New constructor.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/move-tracking-2@1607144 13f79535-47bb-0310-9956-ffa450edef68