On my way to make 'svnmucc' usable for api users, remove some code that is
duplicated from within libsvn_client by creating a svn_client_ctx_t within
svnmucc and initializing the ra sessions from that.

* subversion/svnmucc/svnmucc.c
  (open_tmp_file,
   create_ra_callbacks): Remove functions.
  (execute): Remove arguments that are handled via svn_client_ctx_t.
  (sub_main): Add svn_client_ctx_t, initialize it and pass it.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1550845 13f79535-47bb-0310-9956-ffa450edef68