Make the recently added mtcc libsvn_client api properly obtain a log message
via the svn_client_ctx_t callbacks.

* subversion/libsvn_client/mtcc.c
  (add_commit_items): New function.
  (svn_client_mtcc_commit): Call log function.

* subversion/tests/libsvn_client/mtcc-test.c
  (verify_mtcc_commit): Update caller.
  (test_swap): Add a new test.
  (test_list): Add test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1550799 13f79535-47bb-0310-9956-ffa450edef68