Check, rather than assume, that an existing conflict is a tree-conflict.

* subversion/libsvn_wc/wc_db_update_move.c
  (mark_tree_conflict): Add conflict parameter.
  (tc_editor_add_directory, tc_editor_add_file): Adjust callers.
  (check_tree_conflict): Check existing conflict.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1435098 13f79535-47bb-0310-9956-ffa450edef68