Avoid reading input again if it was closed

After

    commit 0b31b294b7e2295c2df231080db0601f72a242c5
    Author: Peter Wu <lekensteyn@gmail.com>
    Date:   Sat Feb 9 15:15:27 2013 +0100

        Output a colored diff more often

the input file descriptor would be re-opened when closed before the colorization
process. This behavior was observed when starting colordiff without arguments
(interactive mode) and pressing ^D (EOF). Previously, colordiff would exit (as
expected), but after the above commit, it did not, you had to ^D again. This
patch restores the old behavior.