Fix issue #3067, 'subtrees that don't exist at the start or end of a merge
range shouldn't break the merge'

Fix the core problem of issue #3076: Don't try to describe invalid subtrees
to the merge report editor.

* subversion/libsvn_client/merge.c
  (init_rangelist, push_range): New helpers.
  (does_merge_delete_subtree): New helper for filter_merged_revisions()
  when operating on a subtree.
  (prepare_subtree_ranges): New helper for filter_merged_revisions() when
  operating on subtrees.
  (filter_merged_revisions): Use new prepare_subtree_ranges() helper to
  address various issue #3067 problems.  Do away with entry argument
  and instead pass a svn_client__merge_path_t representing the working copy
  target path, along with  another svn_client__merge_path_t representing the
  path's parent if the path is a subtree.  Cascade some additional args from
  calculate_remaining_ranges().
  (calculate_remaining_ranges): Improve doc string.  Tweak signature to get
  svn_client__merge_path_t for target and parent if a subtree.  Update call
  to filter_merged_revisions().
  (populate_remaining_ranges, do_file_merge): Update calls to
  calculate_remaining_ranges().
  
* subversion/tests/cmdline/merge_tests.py
  (dont_merge_revs_into_subtree_that_predate_it): Remove comment about XFail.
  Tweak expected merge notification range output.
  (test_list): Remove XFail from
  dont_merge_revs_into_subtree_that_predate_it.

Patch by: me, kameshj