Destroys .rpmnew files if not connected to a tty

I ran the following command to upgrade my system to Fedora 19, and found that all .rpmnew files were removed.

    fedora-upgrade 2>&1 | tee /root//fedora-upgrade.f19.out

Here's a snippet from the output:

    ...
    Configuration file `/etc/shells'
    -rw-r--r--. 1 root root 60 Jan 21 21:15 /etc/shells
    -rw-r--r--. 1 root root 76 Jun  7 15:31 /etc/shells.rpmnew
     ==> Package distributor has shipped an updated version.
       What would you like to do about it ?  Your options are:
        Y or I  : install the package maintainer's version
        N or O  : keep your currently-installed version
          D     : show the differences between the versions
          M     : merge configuration files
          Z     : background this process to examine the situation
          S     : skip this file
     The default action is to keep your current version.
    *** aliases (Y/I/N/O/D/Z/S) [default=N] ? 
    /sbin/rpmconf: line 104: read: read error: 1: Bad file descriptor
    ...