Finish issue #4151 ("patch for svncopy.pl option --file").

* contrib/client-side/svncopy/svncopy.pl.in
  Teach this script to absorb the --file option processing rather than
  trying to pass the option off verbatim to 'svn' subcommands which
  don't even accept it (such as 'checkout').

Patch by: dario.borreguero{_AT_}gmail.com


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1457920 13f79535-47bb-0310-9956-ffa450edef68