New symmetric merge tests which cover some failing subtree merge use cases.
New symmetric merge tests which cover some failing subtree merge use cases.

* subversion/tests/cmdline/merge_symmetric_tests.py
  (set_up_branch): Import.

  (Merging scenarios to test): Add some new diagrams to the global comments.
 
  (subtree_to_and_fro,
   merge_to_reverse_cherry_subtree_to_merge_to): New.

  (test_list): Add subtree_to_and_fro and
   merge_to_reverse_cherry_subtree_to_merge_to.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion