backport.pl: Add some shell-escaping.

* tools/dist/backport.pl
  (shell_escape, shell_safe_path_or_url, my_tempfile): New helpers.
  (merge, handle_entry): Escape $entry{branch} and @conflicts.
  (merge, edit_string, vote): Use my_tempfile().
  (nominate_main): Outsource validation to new helper (which vote() uses too).
  (Carp): New use().

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570045 13f79535-47bb-0310-9956-ffa450edef68
backport.pl --renormalize:

* tools/dist/backport.pl
  (backport_main): Run a few more checks before renormalizing.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570046 13f79535-47bb-0310-9956-ffa450edef68