Allow gracefull handling of the case where an 'mtcc' is opened on a file or
not existing node, by automatically reparenting one level up when allowed.

* subversion/include/svn_client.h
  (svn_client_mtcc_create): Update api to allow anchoring higher.
  (svn_client_mtcc_get_relpath): Update api to allow anchoring higher.

* subversion/libsvn_client/mtcc.c
  (svn_client_mtcc_create): Check if the anchor exist and handle the case where
    it doesn't.
  (svn_client_mtcc_get_relpath): Allow requesting at least one level higher than
    the passed url. (needed for things like mkdir)

* subversion/tests/libsvn_client/mtcc-test.c
  (make_greek_tree,
   test_mkdir,
   test_mkgreek,
   test_swap,
   test_propset,
   test_update_files,
   test_overwrite): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1550848 13f79535-47bb-0310-9956-ffa450edef68