Strengthen the discussion and checks around passing all children into an add_directory() call.
* subversion/include/svn_editor.h: (...): clarify that children must be mentioned in the parent's add_directory call.
* subversion/libsvn_delta/editor.c: (...): reorder some of the marker declarations and add docs (marker_added_dir, MARKER_ADDED_DIR): new marker (MARK_RELPATH): helper macro for marking stuff in completed_nodes (MARK_COMPLETED, MARK_ALLOW_ADD, MARK_ALLOW_ALTER): use MARK_RELPATH (MARK_ADDED_DIR): new marking helper for nodes from add_directory. another variant for when checking is disabled. (CHECK_UNKNOWN_CHILD): new validation to ensure a child under an added directory was mentioned as a child. plus a variant for when checking is disabled. (check_unknown_child): helper for above (svn_editor_add_directory): mark this specifically as an added dir. use the new CHECK_UNKNOWN_CHILD() checking macro. (svn_editor_add_file, svn_editor_add_symlink, svn_editor_add_absent): use the new CHECK_UNKNOWN_CHILD macro.