Add a bit of cancellation and notification support to the mtcc api.

* subversion/libsvn_client/mtcc.c
  (mtcc_get_origin): Add documentation.
  (commit_file): Add documentation. Report files to the notification handler.
  (commit_directory): Add documentation. Call cancel handler before every
    child. Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1550981 13f79535-47bb-0310-9956-ffa450edef68