Don't mark file as modified when copying (rather than cutting) text.

Original nano commit: #4573 http://svn.savannah.gnu.org/viewvc/trunk/nano/src/cut.c?root=nano&r1=4453&r2=4573
Original nano commit message:
[2]013-04-07 Michael Berg <mike@berg-net.us>
        * do_cut_text - Fix copying (not cutting) text setting Modified state.  Fixes
          issue reported by Benno Schulenberg.