Teach backport.pl to open an interactive subshell.

* tools/dist/backport.pl
  (prompt): Let the caller specify the text.
  (handle_entry): Open a subshell, update calls to prompt().

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1471511 13f79535-47bb-0310-9956-ffa450edef68