Continue migrating functionality from backport.pl to backport.py

* tools/dist/nominate-backport.py
  (subprocess_output): Remove local function affecting...
  (check_local_mods_to_STATUS),
  (get_availid),
  (main): ... these functions, replacing with a call to
    backport.merger.run_svn() or one of its close friends.
  (main): Moving local code figuring out some wc data from here...
* backport/wc.py
  (get_wc_info): ... to here to enable future re-use (parsing additional
    data at the same time).



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1925156 13f79535-47bb-0310-9956-ffa450edef68