backport.pl: parse "--accept=mc" from "Notes:" headers.

* tools/dist/backport.pl
  (parse_entry): Parse "Notes" headers into a ->{accept} optional member.
    While here, consolidate parsing of "Branch:" and "Depends:" too, since
    the previous lazy solution no longer suffices.
  (handle_entry): Include --accept in the prompt.
  (merge): Include --accept in the log message and 'svn merge' arguments.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509590 13f79535-47bb-0310-9956-ffa450edef68