backport.pl interactive mode: Ignore stray keystrokes.

* tools/dist/backport.pl
  (POSIX::isprint, POSIX::isspace): New use()s.
  (prompt): Detect and discard invalid keystrokes.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570047 13f79535-47bb-0310-9956-ffa450edef68
backport.pl conflicts mode:

* tools/dist/backport.pl
  (handle_entry): Fix the check for tree conflicts to look at the right column.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570048 13f79535-47bb-0310-9956-ffa450edef68