Resolve an endless loop uncovered by writing some testcases for move-back
scenarios.

* subversion/libsvn_wc/wc_db.c
  (db_op_copy): Add comment.
  (handle_move_back): Resolve endless loop on certain node states by correctly
    retrieving next row.

* subversion/tests/libsvn_wc/op-depth-test.c
  (move_not_present_variants): New function.
  (test_list): Add move_not_present_variants.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1448755 13f79535-47bb-0310-9956-ffa450edef68