backport.pl: preserve local mods to STATUS

Suggested by: julianfoad

* tools/dist/backport.pl
  (merge): Preserve local mods to STATUS, don't munge it in dry runs.
  (main): Disable check for local mods in interactive mode.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1471538 13f79535-47bb-0310-9956-ffa450edef68